Optical compensation films having positive birefringence for liquid crystal display

ABSTRACT

A method for controlling positive birefringence in an optical compensation film (positive C-plate) having high positive birefringence throughout the wavelength range 400 nm&lt;λ&lt;800 nm is provided. The method includes selecting polymers with optically anisotropic subunits (OASUs) that exhibit the buttressing effect, wherein the OASUs may be disks, mesogens or aromatic rings substituted with birefringence enhancing substituents. The method further includes processing the polymer by solution casting to yield a polymer film with high birefringence without the need for stretching, photopolymerization, or other processes. These optical compensation films may be used in LCDs, particularly IPS-LCDs.

1. FIELD OF THE INVENTION

The invention relates to optical compensation films with positivebirefringence greater than 0.002 throughout the wavelength range of 400nm<λ<800 nm for use in optical devices such as liquid crystal display(“LCD”) devices, optical switches and waveguides where a controlledlight management is desirable. More particularly, the opticalcompensation films are for use in an in-plane switching LCD (“IPS-LCD”).

2. BACKGROUND OF THE INVENTION

LCDs are used as display screens in many common applications includingdigital clocks, microwaves, laptop computers, calculators and otherelectronic devices. LCDs offer advantages over luminescent displaytechnologies such as cathode ray tubes (CRTs) because they are lighter,thinner and require less voltage and power to operate.

LCD screens have good picture quality and contrast when viewed directly,i.e. at an angle that is perpendicular, or normal, to the plane of thescreen. However, picture quality and contrast decline as the viewingangle increases. Image degradation occurs because the LC cell isbirefringent and splits the entering light beam into two light beams(ordinary and extraordinary) that propagate through the LC cell indifferent directions and different speeds. The ordinary andextraordinary rays move in different planes, at different speeds andhave different indices of refraction (n_(o) and n_(e), respectively).The ordinary ray travels in a direction parallel to the direction of theliquid crystals, while the extraordinary ray travels in a directionperpendicular to the direction of the liquid crystals. Since light beamsat different angles experience different retardations, poor imagequality occurs at higher viewing angles. Optical compensation films areused to improve the image quality at high viewing angles by correctingfor the variation between ordinary and extraordinary indices ofrefraction of the light beams passing through the LC cell.

An IPS-LCD device is a type of LCD with LC molecules that lie in-plane,i.e. parallel to the substrate and parallel to each other. An IPS-LCDgenerally includes a liquid crystal (LC) layer having positive ornegative dielectric anisotropy a pair of glass substrates sandwichingthe LC layer, and a pair of polarizing films sandwiching the glasssubstrates and the LC layer together. The LC layer is applied with alateral electric field that is parallel to the substrates to control thedirection of the LC molecules for image display. In the off position,the molecules of liquid crystal lie parallel to the glass substrates andto each other and the cell's electrode pair. When a voltage is appliedacross the electrode pair, the LC molecules can all rotate freelythrough 45° to align themselves with the field, while remaining parallelto the substrates and other molecules above and below them.

Since IPS-LCDs have molecules that are aligned in-plane, light movingthrough the LC parallel to the plane of molecules plane of the has ahigher index of refraction than light moving in the directionperpendicular to the molecules. Thus, light passing through the IPS LCcell has the relationship n_(∥)>>n_(⊥), or n_(e)>>n_(o). In other words,light moving parallel to the LC molecules in the x or y directions on aCartesian plane has a higher index of refraction than light movingperpendicular to the LC molecules in the z direction.

Optical compensation films with positive birefringence (positiveC-plates) are used to compensate for the imbalance created by an IPS-LCDby providing an optical compensation film wherein n_(⊥)>>n_(∥), orn_(o)>>n_(e). The optically anisotropic units in positive C-plates arealigned perpendicularly to the substrate in general so that light movingin the z direction has a higher index of refraction than light moving inthe x or y direction.

Birefringence, Δn, measures the difference between the indices ofrefraction of the ordinary and extraordinary rays:Δn=n _(o) −n _(e) =n _(⊥) −n _(∥)An IPS-LC's cell has a birefringence characterized by n_(∥)>n_(⊥). Thus,the positive C plate needs a birefringence characterized by n_(⊥)>>n_(∥)to compensate the IPS-LC cell. The greater n_(⊥) is compared to n_(∥) ina positive C plate, the greater the difference in their refractiveindices and the higher the birefringence of the compensation film. Highbirefringence in a compensation film creates a more effective C plate.

The concept of a positive C-plate is known in the art as are somecompositions of positive C-plates such as polystyrene. However,polystyrene compensation films are limited because n_(⊥) is onlyslightly higher than n_(∥). Thus, polystyrene films are poorcompensators for LC cells, and a relatively thick polystyrene film isneeded to appreciate any significant LC cell compensation.

Positive C-plates made of poly(vinylcarbazole) are also known in theart. Poly(vinylcarbazole) compensation films have higher birefringencethan polystyrene films, but are photolytically unstable and decomposewhen exposed to light. Thus, poly(vinylcarbazole) compensation films arenot a commercially viable positive C-plate because they are unstable.

Current commercial positive C-plate materials require expensivepost-synthesis processing such as locking liquid crystal molecules intoa perpendicular alignment by photopolymerization. Without thephotopolymerization step, the film would not have the properbirefringence to function as a compensation film.

US Patent Application No. 2005/0200792 A1 discloses an in-planeswitching liquid crystal display comprising a negative biaxialretardation film and a positive C-plate as a viewing angle compensationfilm.

US Patent Application No. 2005/0140900 A1 discloses an IPS-LCDcomprising a positive A-plate and a positive C-plate. No chemicalcomposition of the positive C-plate is disclosed.

US Patent Application No. 2005/0270458 A1 discloses a multilayer opticalcompensation film comprising optically anisotropic first and secondlayers, wherein the second layer includes amorphous polymer with a glasstransition temperature above 160° C., and the indices of refractionsatisfy the relations |n_(x)−n_(y)|<0.001 andn_(z)−(n_(x)+n_(y))/2>0.005.

U.S. Pat. No. 5,189,538 discloses a liquid crystal display comprising afilm having light transmission properties that includes a uniaxiallystretched polymer film with positive intrinsic birefringence.

U.S. Pat. No. 6,115,095 discloses an in-plane switching LCD comprising afirst compensation layer having positive uniaxial, optical anisotropy,and an optical axis extending perpendicularly to the substrate. Nochemical compositions of the compensation layer are disclosed.

U.S. Pat. No. 6,175,400 discloses a broadband cholesteric optical devicehaving a broadband cholesteric layer and a compensator in the form of apositive birefringence film whose optic axis is substantiallyperpendicular to the film.

US Patent Application No. 2005/0037155 discloses a retardation plateobtained by laminating a homeotropic liquid-crystal film and aphotopolymerizable liquid crystalline compound to a stretched filmhaving a retardation function.

US Patent Application No. 2005/0122456 discloses an optical film with asubstrate without a vertical alignment layer and a homeotropic alignmentliquid crystal film layer formed on the substrate. The homeotropicalignment liquid crystal film layer comprises a homeotropically alignedliquid crystal polymer

Thus, there remains a need for an optical compensation film having alarge positive birefringence value without being subject to expensiveprocesses such as stretching, photo irradiation, and heat treatment. Theoptical compensation film should be stable at ambient conditions,optically transparent, have low color, and be easy to apply onto asubstrate.

3. SUMMARY OF THE INVENTION

One object of the invention is to provide polymers for an opticalcompensation film with high positive birefringence throughout thewavelength range of 400 nm<λ<800 nm for use with an LCD as well asmethods for controlling positive birefringence in such a film. The LCDis preferably an IPS-LCD. In an embodiment of the invention, the opticalcompensation film has indices of refraction that satisfy the equations:n _(z) >>n _(x) and n _(z) >>n _(y)n _(⊥) >>n _(∥)

One embodiment of the invention is a method for controlling positivebirefringence in a compensation film for liquid crystal display byselecting a polymer having controlled negative segment birefringence.The polymer may have a polymer segment with a polymer backbone, alight-stable optically anisotropic sub-unit (OASU) attached directly tothe polymer backbone via at least one covalent bond. The polymer segmentmay have negative segment birefringence which is controlled by selectingan OASU such that R/D>about 2.7, where R represents the maximumdimension of the OASU in the direction perpendicular to the direction ofthe vector sum of the at least one covalent bond and D represents thedistance along the polymer backbone between the attaching points of twoneighboring OASUs. The selection of OASU affects the rigidity andlong-range linear corkscrew shape of the polymer backbone such that theaverage orientation of the OASU is perpendicular to the polymerbackbone, and the higher the perpendicularity of the OASUs, the largerthe value of the negative segment birefringence of the polymer segment.The method further involves processing the polymer having a controllednegative segment birefringence (Δn^(s)) by solution casting onto asubstrate, uniaxial stretching, biaxial stretching, or a combinationthereof such that the polymer has a negative segment order parameter(O^(s)) and the polymer film has a positive birefringence (Δn) thatsatisfies the relation Δn=Δn^(s)×O^(s)>0.

That method includes polymers with a moiety of

wherein R₁, R₂, and R₃ are each independently hydrogen atoms, alkylgroups, substituted alkyl groups, or halogens, wherein OASU is adisk-like group or a mesogen, and wherein OASU is attached to thepolymer backbone through a single covalent bond. The method alsoincludes polymers with a moiety of

wherein R₁ and R₃ are each independently hydrogen atoms, alkyl groups,substituted alkyl groups, or halogens, wherein OASU is a disk-like groupor a mesogen, and wherein OASU is attached to the polymer backbonethrough two independent covalent bonds. The OASU that is attached to thepolymer backbone through two independent covalent bonds may be any ofthe following:

Compensation films made through this method are capable of forming anout-of-plane anisotropic alignment upon solvent evaporation withoutbeing subject to heat treatment, photo irradiation, or stretching andmay have a positive birefringence greater than 0.002, 0.005, 0.01, 0.02or 0.03 throughout the wavelength range of 400 nm<λ<800 nm. In oneembodiment, these compensation films may be removed from the substrateupon drying to yield a free-standing film which may be uniaxially orbiaxially stretched. The free-standing film may be attached to asubstrate by lamination with or without stretching.

In one embodiment, the method produces polymers that are soluble in asolvent such as toluene, methyl isobutyl ketone, cyclopentanone, and amixture thereof. In another embodiment, the polymer compositions may beused in a liquid crystal display (LCD) device including an in-planeswitching liquid crystal display device. The LCD may be used as a screenfor a television or computer. The polymer compositions may behomopolymers or copolymers.

The OASU used in this method may be a disk, wherein the disk comprises afused ring structure, which may be an aromatic imide or lactam,naphthalene, anthracene, pyrene, and phthalimide, or any of thefollowing structures, where the disk is attached to the polymer backbonevia a carbon atom on a benzene ring or a nitrogen atom on an imide orlactam group:

In one embodiment, homopolymers may be produced as a reaction product ofany of the following monomers:

Exemplary polymers include poly(N-vinyl-4-tert-butylphthalimide) andpoly(2-vinylnaphthalene). In one embodiment, poly(2-vinylnaphthalene) isprepared by emulsion polymerization so that it may have an averagemolecular weight of greater than 300,000 g/mol.

In one embodiment, compensation films made using the method ofcontrolling birefringence with disk OASUs may be capable of forming anout-of-plane anisotropic alignment upon solvent evaporation withoutbeing subject to heat treatment, photo irradiation, or stretching andmay have a positive birefringence greater than 0.002 or greater than0.005 throughout the wavelength range of 300 nm<λ<800 nm. In anotherembodiment, these compensation films may be soluble in a solvent such astoluene, methyl isobutyl ketone, cyclopentanone, or a mixture thereof.Disk-containing compensation films may also be used in a liquid crystaldisplay device, including an in-plane switching liquid crystal displaydevice. The LCD device may be used as a screen for a television orcomputer.

In another embodiment, the OASU used in the method for controllingbirefringence is a rod-like mesogen. In one embodiment, mesogens havethe structure:R¹-(A¹-Z¹)_(m)-A²-(Z²-A³)_(n)-R²wherein A¹, A², and A³ are each independently aromatic or cycloaliphaticrings. The rings may be all-carbon or heterocyclic, or the rings may beunsubstituted, mono- or poly-substituted with halogen, cyano or nitrogroups or alkyl, alkoxy, or alkanoyl groups having 1 to 8 carbon atoms;wherein Z¹, Z², and Z³ are each independently —COO—, —OOC—, —CO—,—CONH—, —NHCO—, —CH═CH—, —C≡C—, —CH═N—, —N═CH—, —N═N—, —O—, —S—, or asingle bond; wherein R¹ and R² are each independently halogen, cyano,nitro, or alkyl, alkoxy, or alkanoyl groups having 1 to 25 carbon atoms,or are (Z²-A³) as defined above; wherein m is 0, 1, or 2; and wherein nis 1 or 2. Preferably, m is 1 or 2, n is 1 or 2, A² is 1,4-phenylene,and the mesogen is attached to the polymer backbone through A². Alsopreferably, m is 2, n is 2, A² is 1,4-phenylene, and the mesogen isattached to the polymer backbone through A².

The mesogen-containing polymer compositions made by the above method maybe homopolymers or copolymers. In one embodiment, mesogens are any ofthe following structures, wherein the mesogen is attached to the polymerbackbone via a carbon atom on a benzene ring

In another embodiment, the mesogen has any of the following structures,wherein the mesogen is attached to the polymer backbone via a carbonatom on the center 1,4-phenylene:

In another embodiment, mesogen-containing polymers are a reactionproduct of a monomer such as:

wherein the polymer has a positive birefringence greater than about 0.02throughout the wavelength range of 400 nm<λ<800 nm, or from:

In another embodiment, mesogen-containing polymer compositions madeaccording to the method of controlling birefringence may include amoiety in the polymer backbone such as the following structures, whereinR¹, R² and R³ are each independently hydrogen, alkyl group, substitutedalkyl group or halogen:

wherein the polymer has a positive birefringence greater than about 0.01or 0.02 throughout the wavelength range of 400 nm<λ<800 nm.

In another embodiment, mesogen-containing polymer compositions madeaccording to the method for controlling positive birefringence may becapable of forming an out-of-plane anisotropic alignment upon solventevaporation without being subject to heat treatment, photo irradiation,or stretching and may have a positive birefringence greater than 0.002,0.005, 0.01, 0.02 or 0.03 throughout the wavelength range of 400nm<λ<800 nm. In one embodiment, the compensation films may be removedfrom the substrate upon drying to yield a free-standing film, which maybe uniaxially or biaxially stretched. The free-standing film may beattached to a substrate by lamination with or without stretching. In oneembodiment, mesogen-containing polymer compositions made according tothe method for controlling positive birefringence may be soluble intoluene, methyl isobutyl ketone, cyclopentanone, and a mixture thereof.

In yet another embodiment, mesogen-containing polymer compositions madeaccording to the method for controlling positive birefringence are usedin a liquid crystal display device, including an in-plane switchingliquid crystal display device. The LCD may be used as a screen for atelevision or computer.

In yet another embodiment of the invention, there is provided a methodfor controlling positive birefringence in a compensation film for liquidcrystal display by selecting a copolymer having controlled negativesegment birefringence where the copolymer comprises a moiety:

where R¹, R², R³, R⁴, R⁵, and R⁷ are each independently hydrogen atoms,alkyl groups, substituted alkyl groups, or halogens; where R⁶ is a groupwherein R⁶ is a hydrogen atom, alkyl, substituted alkyl, halogen, ester,amide, ketone, ether, cyano, phenyl, epoxy, urethane, urea or opticallyanisotropic subunit (OASU) attached directly to the backbone of aresidue of an ethylenically unsaturated monomer; where disk is anoptically anisotropic subunit (OASU) having a fused ring structurecomprising at least two rings. The disk is attached directly to thecopolymer backbone via at least one covalent bond, and the negativesegment birefringence of the copolymer is controlled by selecting a disksuch that R/D>about 2.7, where R represents the maximum dimension of thedisk in the direction perpendicular to the direction of the vector sumof the at least one covalent bond and D represents the distance alongthe copolymer backbone between the attaching points of disk and R⁶. Theselection of disk affects the rigidity and long-range linear corkscrewshape of the copolymer backbone such that the average orientation of thedisk is perpendicular to the copolymer backbone, and the higher theperpendicularity of the disks, the larger the value of the negativesegment birefringence of the polymer segment. The method furtherinvolves processing the copolymer having a controlled negative segmentbirefringence (Δn^(s)) by solution casting onto a substrate, uniaxialstretching, biaxial stretching, or a combination thereof, such that thecopolymer has a negative segment order parameter (O^(s)) and thecopolymer film has a positive birefringence (Δn) that satisfies therelation Δn=Δn^(s)×O^(s)>0.

In one embodiment, R⁶ may be an OASU such as a disk. The copolymer madeby the method for controlling positive birefringence may also include atleast two different disks. A disk may be an aromatic imide or lactam,naphthalene, anthracene, pyrene, phthalimide, or any of the followingstructures, which are attached to the polymer backbone via a carbon atomon a benzene ring or a nitrogen atom on an imide or lactam group:

In one embodiment, the copolymer made by the method for controllingpositive birefringence is a reaction product of any or a combination ofthe following monomers:

In one embodiment, the copolymer made by the method for controllingpositive birefringence has styrene monomers. In another embodiment, thecopolymer's ethylenically unsaturated monomers include styrene, vinylbiphenyl, methyl methacrylate, butyl acrylate, acrylic acid, methacrylicacid, acrylonitrile, 2-ethylhexyl acrylate, and 4-t-butylstyrene.

In one embodiment, the copolymer made by the method for controllingpositive birefringence is capable of forming an out-of-plane anisotropicalignment upon solvent evaporation without being subject to heattreatment, photo irradiation, or stretching and has a positivebirefringence greater than 0.002 or 0.005 throughout the wavelengthrange of 400 nm<λ<800 nm. In another embodiment, the copolymer may besoluble in toluene, methyl isobutyl ketone, cyclopentanone, and amixture thereof.

In one embodiment, the copolymer made by the method of for controllingpositive birefringence is used in a liquid crystal display device,including an in-plane switching liquid crystal display device. The LCDmay be used as a screen for a television or computer.

In yet another embodiment of the invention there is provided a methodfor controlling positive birefringence in a compensation film for liquidcrystal display by selecting a copolymer having controlled negativesegment birefringence where the copolymer has a moiety:

where R¹, R², R³, R⁴, R⁵, and R⁷ are each independently hydrogen atoms,alkyl groups, substituted alkyl groups, or halogens; where R⁶ is ahydrogen atom, alkyl, substituted alkyl, halogen, ester, amide, ketone,ether, cyano, phenyl, epoxy, urethane, urea or optically anisotropicsubunit (OASU) attached directly to the backbone of the residue of anethylenically unsaturated monomer; and where Mesogen is a rod-likeoptically anisotropic subunit (OASU) attached directly to the polymerbackbone via at least one covalent bond. The negative segmentbirefringence of the copolymer is controlled by selecting a Mesogen suchthat R/D>about 2.7, where R represents the maximum dimension of theMesogen in the direction perpendicular to the direction of the vectorsum of the at least one covalent bond and D represents the distancealong the copolymer backbone between the attaching points of Mesogen andR⁶. The selection of Mesogen affects the rigidity and long-range linearcorkscrew shape of the copolymer backbone such that the averageorientation of the Mesogen is perpendicular to the copolymer backbone,and the higher the perpendicularity of the Mesogens, the larger thevalue of the negative segment birefringence of the copolymer segment.The method further involves processing the copolymer having a controllednegative segment birefringence by solution casting onto a substrate,uniaxial stretching, biaxial stretching, or a combination thereof, suchthat the copolymer has a negative segment order parameter (O^(s)) andthe copolymer film has a positive birefringence (Δn) that satisfies therelation Δn=Δn^(s)×O^(s)>0.

In one embodiment, the mesogen has the structure:R¹-(A¹-Z¹)_(m)-A²-(Z²-A³)_(n)-R²wherein A¹, A², and A³ are each independently aromatic or cycloaliphaticrings, wherein the rings are all-carbon or heterocyclic, wherein therings are unsubstituted, mono- or poly-substituted with halogen, cyanoor nitro groups or alkyl, alkoxy, or alkanoyl groups having 1 to 8carbon atoms; wherein Z¹, Z², and Z³ are each independently —COO—,—OOC—, —CO—, —CONH—, —NHCO—, —CH═CH—, —C≡C—, —CH═N—, —N═CH—, —N═N—, —O—,—S—, or a single bond; wherein R¹ and R² are each independently halogen,cyano, nitro, or alkyl, alkoxy, or alkanoyl groups having 1 to 25 carbonatoms, or are (Z²-A³) as defined above; wherein m is 0, 1, or 2; andwherein n is 1 or 2. Preferably, m is 1 or 2, n is 1 or 2, A² is1,4-phenylene, and the mesogen is attached to the polymer backbonethrough A². More preferrably, m is 2, n is 2, A² is 1,4-phenylene, andthe mesogen is attached to the polymer backbone through A².

In one embodiment, R⁶ may be an OASU such as a mesogen. In anotherembodiment, the copolymer made by the method for controlling positivebirefringence in a compensation film has at least two different mesogengroups. The mesogen may be any of the following structures, wherein themesogen is attached to the polymer backbone via a carbon atom on abenzene ring:

In another embodiment, the mesogen may be any of the followingstructures, wherein the mesogen is attached to the polymer backbone viaa carbon atom on the center 1,4-phenylene:

In one embodiment, the copolymer made by the method for controllingpositive birefringence is a reaction product of monomers, wherein atleast one monomer is any of the following:

wherein the polymer has a positive birefringence greater than about 0.02throughout the wavelength range of 400 nm<λ<800 nm.

In another embodiment, the copolymer made by the method for controllingpositive birefringence in a compensation film is a reaction product ofmonomers, wherein at least one monomer is any of the following:

In another embodiment, the copolymer made by the method for controllingpositive birefringence is a reaction product of monomers, wherein atleast one moiety in the copolymer backbone is any of the followingstructures, wherein R¹, R² and R³ are each independently hydrogen, alkylgroup, substituted alkyl group or halogen:

wherein the polymer has a positive birefringence greater than about 0.01throughout the wavelength range of 400 nm<λ<800 nm.

In one embodiment, the copolymer made by the method for controllingpositive birefringence is a reaction product of monomers, wherein atleast one moiety in the copolymer backbone is any of the followingstructures, and wherein R¹, R² and R³ are each independently hydrogen,alkyl group, substituted alkyl group or halogen:

wherein the polymer has a positive birefringence greater than about 0.02throughout the wavelength range of 400 nm<λ<800 nm.

In another embodiment, the copolymer made by the method for controllingpositive birefringence includes styrene monomers. In another embodiment,the ethylenically unsaturated monomer of the copolymer may be styrene,vinyl biphenyl, methyl methacrylate, butyl acrylate, acrylic acid,methacrylic acid, acrylonitrile, 2-ethylhexyl acrylate, or4-t-butylstyrene.

In one embodiment, the copolymer made by the method for controllingpositive birefringence may be capable of forming an out-of-planeanisotropic alignment upon solvent evaporation without being subject toheat treatment, photo irradiation, or stretching and has a positivebirefringence greater than 0.002, 0.005, 0.01, 0.02 or 0.03, throughoutthe wavelength range of 400 nm<λ<800 nm. In one embodiment, thecopolymer may be removed from the substrate upon drying to yield afree-standing film which may be uniaxially or biaxially stretched. Thefree-standing film may be attached to a substrate by lamination with orwithout stretching.

In one embodiment, the copolymers made by the method for controllingpositive birefringence are soluble in a solvent such as toluene, methylisobutyl ketone, cyclopentanone, and a mixture thereof. In anotherembodiment, the copolymers are used in a liquid crystal display deviceincluding an in-plane switching liquid crystal display device. The LCDmay be used as a screen for a television or computer.

Yet another object of the invention provides a method for controllingpositive birefringence in a compensation film for liquid crystal displayby selecting a polymer having controlled negative segment birefringence.The polymer may have a polymer segment having a polymer backbone and alight-stable optically anisotropic sub-unit (OASU) comprising anaromatic ring and at least one birefringence enhancing substituent (BES)attached to the aromatic ring, wherein the Ar-BES is attached directlyto the polymer backbone via at least one covalent bond. The polymersegment may have negative segment birefringence which is controlled byselecting an Ar-BES such that R/D is at least about 2.6, wherein Rrepresents the maximum dimension of the Ar-BES in the directionperpendicular to the direction of the vector sum of the at least onecovalent bond and D represents the distance along the polymer backbonebetween the attaching points of two neighboring Ar-BESs. The selectionof Ar-BES affects the rigidity and long-range linear corkscrew shape ofthe polymer backbone such that the average orientation of the Ar-BES isperpendicular to the polymer backbone, and the higher theperpendicularity of the Ar-BESs, the larger the value of the negativesegment birefringence of the polymer segment. The method furtherinvolves processing the polymer having a controlled negative segmentbirefringence (Δn^(s)) by solution casting onto a substrate, uniaxialstretching, biaxial stretching, or a combination thereof, wherein thepolymer has a negative segment order parameter (O^(s)) and the polymerfilm has a positive birefringence (Δn) that satisfies the relationΔn=Δn^(s)×O^(s)>0.

In one embodiment, the method for controlling positive birefringenceincludes polymers with a moiety of

wherein R₁, R₂, and R₃ are each independently hydrogen atoms, alkylgroups, substituted alkyl groups, or halogens; Ar is an aromatic ring;and BES represents at least one birefringence enhancing substituent.

In one embodiment, the method for controlling positive birefringenceincludes making homopolymers. In another embodiment, the method includescompensation films that are capable of forming an out-of-planeanisotropic alignment upon solvent evaporation without being subject toheat treatment, photo irradiation, or stretching and has a positivebirefringence greater than 0.002, 0.005 or 0.01 throughout thewavelength range of 400 nm<λ<800 nm.

In one embodiment, polymer films made by the method for controllingpositive birefringence are removed from the substrate upon drying toyield a free-standing film, which may be uniaxially or biaxiallystretched to further increase birefringence. The free-standing film mayalso be attached to a substrate by lamination with or withoutstretching.

In another embodiment, the method for controlling positive birefringenceproduces polymers that are soluble in a solvent such as toluene, methylisobutyl ketone, cyclopentanone, and a mixture thereof. In oneembodiment, the compensation films are used in a liquid crystal displaydevice including an in-plane switching liquid crystal display device.The LCD may be used as a screen for a television or computer.

In another embodiment, the method for controlling birefringence furtherincludes controlling the degree of substitution of the BES by adjustingstarting amounts of BES in a reaction mixture. The degree ofsubstitution of the BES may be greater than 0.5. In one embodiment, BESmay be nitro-, bromo-, iodo-, cyano- and phenyl-, and is preferablynitro- or bromo-.

In one embodiment, aromatic rings of the Ar-BES compensation films madeby the method for controlling positive birefringence in a compensationfilm for liquid crystal display may be benzene, biphenyl, naphthalene,anthracene, phenanthrene, naphthacene, pentacene, or triphenyl. In apreferred embodiment, the aromatic ring is benzene. In another preferredembodiment, BES is nitro- or bromo- and the aromatic ring is benzene,biphenyl, naphthalene, anthracene, phenanthrene, naphthacene, pentacene,or triphenyl.

In yet another embodiment, the compensation film made by the method forcontrolling positive birefringence in a compensation film for liquidcrystal display is poly(nitrostyrene). The poly(nitrostyrene) may have apositive birefringence greater than 0.007 throughout the wavelengthrange of 400 nm<λ<800 nm. The poly(nitrostyrene) may have a degree ofsubstitution greater than 0.5 or 0.7 for the nitro group. The BES mayhave some para-nitro groups or nearly all para-nitro groups. In oneembodiment, the BES has nearly all para-nitro groups and a degree ofsubstitution greater then 0.5 or 0.7 for the nitro group.

In yet another embodiment, the compensation film made by the method forcontrolling positive birefringence in a compensation film for liquidcrystal display is poly(bromostyrene). The poly(bromostyrene) may have apositive birefringence greater than 0.005 throughout the wavelengthrange of 400 nm<λ<800 nm. The poly(bromostyrene) may have a degree ofsubstitution greater than 0.5 or 0.7 for the bromo group. The BES mayhave some para-bromo groups or nearly all para-bromo groups. In oneembodiment, the BES has nearly all para-bromo groups and a degree ofsubstitution greater then 0.5 or 0.7 for the bromo group.

Yet another object of the invention is to provide a method forcontrolling positive birefringence in a compensation film for liquidcrystal display by selecting a copolymer having controlled negativesegment birefringence. The copolymer may have a moiety such as:

wherein R¹, R², R³, R⁴, R⁵, and R⁷ are each independently hydrogenatoms, alkyl groups, substituted alkyl groups, or halogens; wherein R⁶is a hydrogen atom, alkyl, substituted alkyl, halogen, ester, amide,ketone, ether, cyano, phenyl, epoxy, urethane, urea or opticallyanisotropic subunit (OASU) attached directly to the backbone of theresidue of an ethylenically unsaturated monomer; wherein Ar-BES is anaromatic ring (Ar) substituted with at least one birefringence enhancingsubstituent (BES), and wherein Ar-BES is attached directly to thecopolymer backbone via at least one covalent bond. The negative segmentbirefringence of the copolymer is controlled by selecting an Ar-BES suchthat R/D is at least about 2.6, wherein R represents the maximumdimension of the Ar-BES in the direction perpendicular to the directionof the vector sum of the at least one covalent bond and D represents thedistance along the copolymer backbone between the attaching points ofAr-BES and R⁶. The selection of Ar-BES affects the rigidity andlong-range linear corkscrew shape of the copolymer backbone such thatthe average orientation of the Ar-BES is perpendicular to the copolymerbackbone, and the higher the perpendicularity of the Ar-BESs, the largerthe value of the negative segment birefringence of the copolymer. Themethod further involves processing the copolymer having a controllednegative segment birefringence (Δn^(s)) by solution casting onto asubstrate, uniaxial stretching, biaxial stretching, or a combinationthereof. The resulting copolymer has a negative segment order parameter(O^(s)) and the copolymer film has a positive birefringence (Δn) thatsatisfies the relation Δn=Δn^(s)×O^(s)>0.

In one embodiment, R⁶ is an OASU. In another embodiment, R⁶ is anAr-BES. The copolymer may also comprise at least two different Ar-BESgroups. In one embodiment, the ethylenically unsaturated monomer may bestyrene, vinyl biphenyl, methyl methacrylate, butyl acrylate, acrylicacid, methacrylic acid, acrylonitrile, 2-ethylhexyl acrylate, or4-t-butylstyrene. In another embodiment, at least one monomer of thecopolymer is styrene.

In another embodiment, the compensation film formed by the method forcontrolling positive birefringence is capable of forming an out-of-planeanisotropic alignment upon solvent evaporation without being subject toheat treatment, photo irradiation, or stretching and has a positivebirefringence greater than 0.002, greater than 0.005 or greater than0.01, throughout the wavelength range of 400 nm<λ<800 nm.

In one embodiment, the compensation film formed by the method forcontrolling birefringence is removed from the substrate upon drying toyield a free-standing film, which may be uniaxially or biaxiallystretched. The free-standing film may be attached to a substrate bylamination with or without stretching.

In one embodiment, the compensation film made using the method forcontrolling birefringence is soluble in a solvent selected from thegroup consisting of toluene, methyl isobutyl ketone, cyclopentanone, anda mixture thereof. In another embodiment, the BES is nitro- or bromo-.

In another embodiment, the compensation film made using the method forcontrolling birefringence is used in a liquid crystal display deviceincluding an in-plane switching liquid crystal display device.

The method of controlling birefringence may further include controllingthe degree of substitution of the BES. In one embodiment, the degree ofsubstitution of the BES is greater than 0.5. In another embodiment, BESis nitro-, bromo-, iodo-, cyano- or phenyl-. Preferably, BES is nitro-or bromo-. In one embodiment, the aromatic ring is benzene, biphenyl,naphthalene, anthracene, phenanthrene, naphthacene, pentacene, ortriphenyl. Preferably, the aromatic ring is benzene. In one embodiment,BES is nitro- or bromo- and the aromatic ring is benzene, biphenyl,naphthalene, anthracene, phenanthrene, naphthacene, pentacene, ortriphenyl.

In one embodiment, the polymer made using the method of controllingbirefringence is poly(nitrostyrene-co-styrene). Thepoly(nitrostyrene-co-styrene) may have a degree of substitution greaterthan 0.5 for the nitro group. The poly(nitrostyrene-co-styrene) may havea degree of substitution greater than 0.7 for the nitro group. The BESof poly(nitrostyrene-co-styrene) may have some para-nitro groups ornearly all para-nitro groups. In one embodiment, thepoly(nitrostyrene-co-styrene) has nearly all para-nitro groups and adegree of substitution greater than 0.5 for the nitro group. In anotherembodiment, the poly(nitrostyrene-co-styrene) has nearly all para-nitrogroups and a degree of substitution greater than 0.7 for the nitrogroup.

In one embodiment, the polymer made using the method of controllingbirefringence is poly(bromostyrene-co-styrene). Thepoly(bromostyrene-co-styrene) may have a degree of substitution greaterthan 0.5 for the bromo group. The poly(bromostyrene-co-styrene) may havea degree of substitution greater than 0.7 for the bromo group. The BESof poly(bromostyrene-co-styrene) have some para-bromo groups or nearlyall para-bromo groups. In one embodiment, thepoly(bromostyrene-co-styrene) has a nearly all para-bromo groups degreeof substitution greater than 0.5 for the bromo group or greater than 0.7for the bromo group.

4. DESCRIPTION OF THE DRAWINGS

FIG. 1 is a depiction of several OASUs and the frameworks forcalculating the buttressing factor for these OASUs. FIGS. 1 a and 1 bdepict the OASU of polystyrene FIGS. 1 c-1 d depict the disk OASU of,poly(2-vinyl naphthalene). FIGS. 1 e-1 f depict the disk OASU ofpoly(1-vinyl naphthalene). FIGS. 1 g-1 h depict the disk OASU ofpoly(vinylpyrene). FIG. 1 i depicts the mesogen OASU, ofpoly[2,5-bis(p-alkoxyphenyl)styrene].

FIG. 2 is a depiction of the framework for calculating the buttressingfactor for polystyrene.

FIG. 3( a) is a side view depiction of a buttressed rod-likemesogen-jacketed polymer chain. FIG. 3( b) is an end view depiction of abuttressed rod-like mesogen-jacketed polymer chain.

FIG. 4 is a table depicting O^(OASU) and Δn^(OASU) parameters forpolymers with disk-like and rod-like OASUs.

FIG. 5 is a depiction of the stages of solvent evaporation duringsolution casting.

FIG. 6 is a plot showing how birefringence varies with the degree ofsubstitution of Ar-BESs with nitro groups.

FIG. 7 is a depiction of the framework for calculating the buttressingfactor for poly(2-vinyl naphthalene).

FIG. 8 is a depiction of the framework for calculating the buttressingfactor for poly(vinylpyrene).

FIG. 9 is a depiction of the framework for calculating the buttressingfactor for poly[2,5-bis(p-alkoxyphenyl)styrene].

5. DETAILED DESCRIPTION OF THE INVENTION

In one example embodiment of the invention is an optical compensationfilm with positive birefringence greater than 0.002 throughout thewavelength range of 400 nm<λ<800 nm and may form an out-of-planeanisotropic alignment upon solvent evaporation when made by solutioncasting. Accordingly, the optical compensation films of the inventionmay be aligned anisotropically such that the net optical axis of arod-like OASU (in the rod-direction) is out-of-plane (where out-of-planeincludes but is not limited to optical axes that are perpendicular tothe plane), and the net optical axis of a disk-like or Ar-BES OASU (inthe disk normal direction) is in-plane (where in-plane includes but isnot limited to optical axes that are parallel to the plane). The opticalcompensation films of the invention may be used as part of a liquidcrystal display (LCD) device, particularly an in-plane switching (IPS)LCD. The LCD may be used in electronic devices with display screensincluding, but not limited to, televisions, computers, cell phone,clocks, microwaves and calculators.

The polymer film with high positive birefringence has a moietycontaining a light stable OASU in the polymer backbone. The OASU may beattached directly to the polymer backbone through one covalent bond sothe moiety has the general formula:

wherein R¹, R², and R³ are each independently hydrogen atoms, alkylgroups, substituted alkyl groups, or halogens, and OASU is an opticallyanisotropic sub-unit. The OASU may also be attached directly to thepolymer backbone through two independent covalent bonds so the moietyhas the general formula:

wherein R¹, R², and R³ are each independently hydrogen atoms, alkylgroups, substituted alkyl groups, or halogens, and OASU is an opticallyanisotropic sub-unit. The covalent bond provides a direct connectionbetween the OASU and the polymer backbone that other atoms are notpositioned along the covalent bond, which would make the connectionbetween the OASU and the polymer backbone indirect.

The polymer film may be a homopolymer or a copolymer. The copolymer mayhave one or more moieties containing an OASU attached directly to thepolymer backbone through at least one covalent bond. The description ofthe invention applies to any OASU-containing homopolymer or copolymerwith any combination of moieties. As used herein, the term “polymer”refers to homopolymers and copolymers.

The OASU may be disk-like, rod like (mesogen), or aromatic rings (Ar)substituted with birefringence enhancing substituents (BES). In apreferred embodiment, the OASU is oriented perpendicular to the polymerbackbone, and the value of the positive birefringence of the polymerfilm increases with increasing perpendicularity of the OASUs.

The polymer solutions may advantageously form an out-of-planeanisotropic alignment upon solvent evaporation and solution film castingwithout being subject to heat treatment, photo irradiation, or astretching process (although one or a combination of these processes maybe used to further entrance birefringence). The resulting buttressedpolymer films are stable at ambient conditions, have high positivebirefringence and may be inexpensive to produce. Positive birefringenceis defined as n_(z)>(n_(x)+n_(y))/2, wherein n_(x) and n_(y) representin-plan refractive indexes, and n_(z) represents the thickness-directionrefractive index of the film. These polymers and the opticalcompensation films made therefrom, each has positive birefringencegreater than 0.002 throughout the wavelength range of 400 nm<λ<800 nmwithout being subject to heat treatment, photo irradiation, orstretching. However, in certain embodiments these processes may be usedto further enhance positive birefringence. In preferred embodiments, thecompensation films may have birefringence greater than 0.005, 0.01, 0.02or 0.03 throughout the wavelength range of 400 nm<λ<800 nm.

Birefringence (Δn) may be measured by determining the birefringence of afilm over a wavelength range of about 300 nm to about 800 nm atdifferent increments. Alternatively, birefringence of a film may bemeasured at 633 nm as is customary in the art. Reference to Δn at 633 nmis customary because birefringence at wavelengths<633 nm is generallyhigher than birefringence at 633 nm, and birefringence atwavelengths>633 nm is generally the same as or slightly lower thanbirefringence at 633 nm. Thus, birefringence at 633 nm is understood inthe art as indicating that birefringence throughout 300 nm<λ<800 nm isgreater than or approximately the same as the birefringence at 633 nm.

In one example embodiment of the invention, the OASU is a disk. The diskmay be attached directly to the polymer backbone through one covalentbond so the moiety has the general formula:

in the polymer backbone, wherein R¹, R², and R³ are each independentlyhydrogen atoms, alkyl groups, substituted alkyl groups, or halogens. Thedisk may also be attached directly to the polymer backbone through twoindependent covalent bonds. The covalent bond may be a carbon-carbon orcarbon-nitrogen bond. For example, disks may be attached to the polymerbackbone via a carbon or nitrogen atoms, such as the carbon atom on abenzene ring or the nitrogen atom on an imide or lactam. Thedisk-containing polymer has a positive birefringence greater than 0.002throughout the wavelength range of 400 nm<λ<800 nm without being subjectto heat treatment, photo irradiation, or stretching. The disk-containingpolymer film may be made by solution casting, and may form anout-of-plane anisotropic alignment upon solvent evaporation. In apreferred embodiment, the positive birefringence is greater than about0.005 throughout the wavelength range of 400 nm<λ<800 nm.

The polymer film may be a homopolymer or copolymer with one or moremoieties containing a disk attached directly to the polymer backbonethrough at least one covalent bond. The copolymer may have a moiety withthe general structure in the polymer backbone:

wherein R¹, R², R³, R⁴, R⁵, and R⁷ are each independently hydrogenatoms, alkyl groups, substituted alkyl groups, or halogens; wherein R⁶is a hydrogen atom, alkyl group, substituted alkyl group, halogen,ester, amide, ketone, ether, cyano, phenyl, epoxy, urethane, urea, oroptically anisotropic subunit (OASU) attached directly to the backboneof a residue of an ethylenically unsaturated monomer. In one embodiment,R⁶ is a different disk. In another embodiment, R⁶ is a benzene ring. Thedisk may also be attached to a copolymer backbone by two covalent bonds.

The disk usually has a size greater than a benzene ring. The disk isusually bulky. In one embodiment, the disk group has a fused ringstructure. The “fused ring” structure may be understood to have two ormore individual rings that are connected by sharing at least one oftheir sides. Each individual ring in the fused ring may be substitutedor unsubstituted and is preferably a six- or five-membered ring, whichmay be all-carbon or heterocyclic. Individual rings in a fused ring maybe aromatic or aliphatic. Preferred individual rings in a fused ringinclude, but are not limited to, aromatic rings and substituted aromaticrings, lactam ring and rings based on aromatic imide such as phthalimideand substituted phthalimide. The disk group is stable at ambientconditions and thus suitable for use in an optical compensation film foran LCD.

Representatives and illustrative examples of disk groups include, butare not limited to, naphthalene, anthracene, phenanthrene, naphthacene,pyrene, pentacene, phthalimide, and the like as shown in the followingchemical structures:

As one skilled in the art will recognize, polymer compositionscomprising moieties with disk groups may be prepared by polymerizationof a disk-containing monomer having a vinyl group attached directly toeither a carbon or a nitrogen atom on the fused ring. Suchdisk-containing monomers with polymerizable vinyl groups include, butare not limited to, the following compounds:

Polymer compositions comprising moieties with disk groups may also beprepared by copolymerization of a disk-containing monomer with one ormore ethylenically unsaturated monomers. Such ethylenically unsaturatedmonomers that may be used to copolymerize with disk-containing monomersinclude, but are not limited to, one or more of methyl acrylate, methylmethacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, butylmethacrylate, isobutyl acrylate, isobutyl methacrylate, ethylhexylacrylate, 2-ethylhexyl methacrylate, 2-ethylhexyl acrylate, isoprene,octyl acrylate, octyl methacrylate, iso-octyl acrylate, iso-octylmethacrylate, trimethylpropyl triacrylate, styrene, α-methyl styrene,nitrostyrene, bromostyrene, iodostyrene, cyanostyrene, chlorostyrene,4-t-butylstyrene, 4-methylstyrene, vinyl biphenyl, vinyl triphenyl,vinyl toluene, chloromethyl styrene, acrylic acid, methacrylic acid,itaconic acid, crotonic acid, maleic anhydride, glycidyl methacrylate,carbodiimide methacrylate, C₁-C₁₈ alkyl crotonates, di-n-butyl maleate,di-octylmaleate, allyl methacrylate, di-allyl maleate, di-allylmalonate,methyoxybutenyl methacrylate, isobornyl methacrylate, hydroxybutenylmethacrylate, hydroxyethyl(meth)acrylate, hydroxypropyl(meth)acrylate,acetoacetoxy ethyl methacrylate, acetoacetoxy ethyl acrylate,acrylonitrile, vinyl chloride, vinylidene chloride, vinyl acetate, vinylethylene carbonate, epoxy butene, 3,4-dihydroxybutene,hydroxyethyl(meth)acrylate, methacrylamide, acrylamide, butylacrylamide, ethyl acrylamide, diacetoneacrylamide, butadiene, vinylester monomers, vinyl(meth)acrylates, isopropenyl(meth)acrylate,cycloaliphaticepoxy(meth)acrylates, ethylformamide,4-vinyl-1,3-dioxolan-2-one, 2,2-dimethyl-4 vinyl-1,3-dioxolane,3,4-di-acetoxy-1-butene, and monovinyl adipate t-butylaminoethylmethacrylate, dimethylaminoethyl methacrylate, diethylaminoethylmethacrylate, N,N-dimethylaminopropyl methacrylamide,2-t-butylaminoethyl methacrylate, N,N-dimethylaminoethyl acrylate,N-(2-methacryloyloxy-ethyl)ethylene urea, andmethacrylamido-ethylethylene urea. Further monomers are described in TheBrandon Associates, 2nd edition, 1992 Merrimack, N.H., and in Polymersand Monomers, the 1966-1997 Catalog from Polyscience, Inc., Warrington,Pa., U.S.A.

Polymerization may be carried out by a method known in the art such asbulk, solution, emulsion, or suspension polymerization. The reaction maybe free radical, cationic, anionic, zwitterionic, Ziegler-Natta, or atomtransfer radical type of polymerization. Emulsion polymerization is apreferred method of polymerization when a particularly high molecularweight is desirable. A high molecular weight polymer may lead to betterfilm quality and higher positive birefringence.

Solution film casting may be done with disk containing polymer, apolymer solution comprising a blend of disk-containing polymer withother polymers, or a copolymer of disk-containing monomer with othermonomers, the latter two being advantageous because they may improvefilm quality and lower cost. Polymer solutions may further contain otheringredients such as other polymers or additives.

Depending on the particular disk structure and polymer or polymer blendcomposition, the disk-containing polymers may be soluble in, forexample, toluene, methyl isobutyl ketone (MIBK), methyl ethyl ketone(MEK), cyclopentanone, N,N-dimethylformamide, or mixtures thereof.Preferred solvents are toluene and MIBK.

In another example embodiment of the invention, the OASU is an aromaticring (Ar) substituted with birefringence enhancing substituents (BES).BES could also be substituents on disk or mesogen OASUs. The Ar-BES mayalso be a fused aromatic ring substituted with BES. The Ar-BES may beattached directly to the polymer backbone through one covalent bond sothe moiety has the general formula:

in the polymer backbone, wherein R¹, R², and R³ are each independentlyhydrogen atoms, alkyl groups, substituted alkyl groups, or halogens. TheAr-BES may also be attached directly to the polymer backbone through twoindependent covalent bonds. The degree of substitution of the aromaticring with BES is at least 0.1, but it may also be higher. The covalentbond may be a carbon-carbon or carbon-nitrogen bond. The Ar-BEScontaining polymer has a positive birefringence greater than 0.002throughout the wavelength range of 400 nm<λ<800 nm without being subjectto heat treatment, photo irradiation, or stretching. TheAr-BES-containing polymer film may be made by solution casting, and mayform an out-of-plane anisotropic alignment upon solvent evaporation. TheAr-BES preferrably has a positive birefringence greater than 0.005, andmore preferrably has a positive birefringence greater than 0.01throughout the wavelength range of 400 nm<λ<800 nm.

The polymer film may be a homopolymer or copolymer with one or moremoieties containing an Ar-BES attached directly to the polymer backbonethrough one covalent bond. The copolymer may have a moiety with thegeneral structure in the polymer backbone:

wherein R¹, R², R³, R⁴, R⁵, and R⁷ are each independently hydrogenatoms, alkyl groups, substituted alkyl groups, or halogens; wherein R⁶is a hydrogen atom, alkyl group, substituted alkyl group, halogen,ester, amide, ketone, ether, cyano, phenyl, epoxy, urethane, urea, oroptically anisotropic subunit (OASU) attached directly to the backboneof the residue of an ethylenically unsaturated monomer. In oneembodiment, R⁶ is a different Ar-BES. In another embodiment, R⁶ is abenzene ring.

The degree of substitution (DS) of BES on the aromatic ring refers tothe average number of BES on one aromatic ring in a polymer composition.Thus, DS=1 when, on average, each aromatic ring is substituted with oneBES. DS may also be greater than one when, on average, each aromaticring is substituted with more than one BES. DS is preferably greaterthan 0.3, more preferably greater than 0.5, and most preferably greaterthan 0.7. The DS of BES is directly related to the polymer'sbirefringence. Thus, Δn may be manipulated by varying the DS. Thesolubility of the polymer can also dependent on the DS and be optimizedaccordingly. The DS can be readily manipulated by one of ordinary skillin the art, for example, by adjusting the starting amounts of BES.

In one embodiment, the Ar-BES-containing polymer is apoly(vinylaromatic), i.e. a polymer resulting from polymerization of thevinyl group on an aromatic ring. The poly(vinylaromatic) also has atleast one BES. Poly(vinylaromatic) with BES advantageously exhibitsexceptionally high birefringence values, is soluble in a variety oforganic solvents, and may be used to prepare an optical compensationfilm by solution casting onto a substrate. The solubility andbirefringence of poly(vinyl aromatics) of the invention can becontrolled by incorporating certain BESs and by adjusting their degreeof substitutions (DSs) of the aromatic rings of the polymers. This ishighly desirable since an LCD device typically contains multi-layers ofmaterials having different solubility in a variety of solvents and alayer can only be coated with a polymer solution that does not dissolvethis specific layer. Thus, the ability to control the solubility andbirefringence of a polymer allows the optical film of the presentinvention to be cast on a specific layer (or substrate) for LCDfabrication to achieve the desirable order of the layers in the device.

Representatives and illustrative examples of aromatic groups include,but are not limited to, benzene, biphenyl, naphthalene, anthracene,phenanthrene, naphthacene, pyrene, pentacene, triphenyl, and the like.Preferably, the aromatic ring is benzene, biphenyl or naphthalene. Mostpreferably, the aromatic ring is benzene.

BES is a group that in general is bulky and/or capable of increasing thepolarizability of the disk groups' aromatic ring on poly(vinylaromatic). A polymer may contain different BES groups on differentaromatic rings within the same polymer molecule or different BES groupson the same aromatic ring. Representatives and illustrative examples ofBES include, but are not limited to, NO₂, Br, I, CN, and phenyl.Preferably, BES substituents are NO₂, Br, I, and CN. Most preferably,BES is NO₂ or Br.

BES may be attached to an aromatic ring such as benzene at any availableposition including the positions that are para, ortho or meta to theethylene moiety. A polymer composition may also have BESs that are indifferent positions on different aromatic rings. In a preferredembodiment, the BES is para to the ethylene moiety. BES may also bemostly at the para position with some BES at the ortho and/or metapositions.

Representatives and illustrative examples of polymer compositions ofBES-substituted aromatic polymers include, but are not limited to,poly(nitrostyrene), poly(bromostyrene), substituted poly(nitrostyrene),substituted poly(bromostyrene), copolymers of nitrostyrene orbromostyrene, and copolymer of substituted nitrostyrene or bromostyrene.Preferably, the polymer composition is poly(nitrostyrene),poly(bromostyrene), a copolymer thereof, or a mixture thereof.

Poly(nitrostyrene), poly(bromostyrene) and copolymers thereof may besubstituted with one or more nitro or bromo BESs, respectively. Thedegree of substitution for bromo or nitro BES is preferrably at least0.5 and more preferrably at least 0.7. However, the degree ofsubstitution may also be higher or lower in the range 0<DS<1. Also, DSmay be greater than one. The nitro or bromo substituent may be attachedto the benzene ring at any available position including the positionsthat are para, ortho or meta to the ethylene moiety. In a preferredembodiment, the nitro or bromo BES is para to the ethylene moiety. Thus,preferred polymers include poly(4-nitrostyrene),poly(4-nitrostyrene-co-styrene), poly(4-bromostyrene) andpoly(4-bromostyrene-co-styrene). As one of skill in the art willrecognize, when these preferred polymers are prepared from 4-nitro- or4-bromostyrene monomers, the nitro or bromo groups, respectively, willalways be at the para position.

As one of skill in the art will recognize, poly(nitrostyrene) may beprepared by nitration of polystyrene in the presence of a mixed acid ofHNO₃ and H₂SO₄ as disclosed in Philippides, A., et al., Polymer (1993),34(16), 3509-13; Fernandez, M. J., et al., Polymer Degradation andStability (1998), 60(2-3), 257-263; Cowie, J. M. G., et al., EuropeanPolymer Journal (1992), 28(2), 145-8; and Al-Najjar, Mohammed M, et al.,Polymer Engineering and Science (1996), 36(16), 2083-2087. Nitration ofpolystyrene can be carried out in the presence of an organic solventsuch as nitrobenzene, 1,2-dichloroethane, 3-nitrotoluene, carbontetrachloride, chloroform, methylene chloride, carbon disulfide,N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrolidone, or amixture thereof. Preferred solvents are nitrobenzene and a 3:1 mixtureof nitrobenzene and 1,2-dichloroethane. Copolymers of nitrostyrene maybe prepared by nitration of a copolymer of styrene such aspoly(styrene-co-acrylonitrile), poly(styrene-co-4-t-butylstyrene), andpoly(styrene-co-methyl methacrylate). They can also be prepared bycopolymerization of nitrostyrene with other ethylenically unsaturatedmonomers such as methyl methacrylate, acrylonitrile, 4-t-butylstyrene,4-methylstyrene, butyl acrylate, and acrylic acid. Poly(nitrostyrene)can also be prepared by polymerization of nitrostyrene monomer asdisclosed in Philippides, A. et al., Polymer (1994), 35(8), 1759-63; andJonquieres, A. et al., Polymer Bulletin (Berlin), (1994), 33(4), 389-95.Trifluoroacetic anhydride and trifluoroacetic acid may be used withnitric acid as the nitration agent. Inorganic nitrate salts such asNH₄NO₃, NaNO₃, KNO₃, and AgNO₃ may also be used with trifluoroaceticanhydride as the nitration agent as disclosed in Grivello, J. V., J.Org. Chem. (1981), 46, 3056-3060.

The poly(nitrostyrene) polymers prepared in this invention are solublein toluene, methyl isobutyl ketone (MIBK), methyl ethyl ketone (MEK),cyclopentanone, N,N-dimethylformamide or a mixture thereof depending onthe degree of substitution of the nitro group. Preferred solvents forfilm casting poly(nitrostyrene) are toluene and MIBK or a mixturethereof.

As one of skill in the art will recognize, poly(bromostyrene) may beprepared by bromination of polystyrene in the presence of bromine and aLewis acid catalyst such as AlCl₃, FeCl₃, AlBr₃, FeBr₃, SbCl₅, ZrCl₄,Sb₂O₃, and the like, as disclosed in U.S. Pat. Nos. 5,677,390 and5,532,322, which are incorporated by reference in their entirety. It mayalso be prepared by reaction of polystyrene with n-butyllithium-TMEDAcomplex followed by bromine quenching as disclosed in Farrall, M. J. andFrechet, M. J., Macromolecules, Vol. 12; p. 426, (1979). Similar topoly(nitrostyrene), poly(bromostyrene) may also be prepared bypolymerization of bromostyrene monomer as described in Farrall, M. J.and Frechet, M. J., Macromolecules, Vol. 12; p. 426, (1979). Likewise,copolymers of bromostyrene may also be prepared as described previouslyfor poly(nitrostyrene). Bromination of polystyrene can be carried out inthe presence of an organic solvent such as, for example,1,2-dichloroethane, nitrobenzene, 3-nitrotoluene, carbon tetrachloride,chloroform, methylene chloride, carbon disulfide, N,N-dimethylformamide,N,N-dimethylacetamide, N-methylpyrrolidone, or a mixture thereof.Preferred solvents are 1,2-dichloroethane, carbon tetrachloride, andchloroform.

The poly(bromostyrene) polymers prepared in this invention are solublein toluene as well as in cyclopentanone even with high degrees ofsubstitution. This is particularly useful for coating a TAC substratesince toluene will not have a detrimental effect on the TAC film.

Polymer compositions comprising moieties with Ar-BES may also beprepared by copolymerization of an Ar-BES-containing monomer with one ormore ethylenically unsaturated monomers. Such ethylenically unsaturatedmonomers that may be used to copolymerize with disk-containing monomersinclude, but are not limited to, one or more of methyl acrylate, methylmethacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, butylmethacrylate, isobutyl acrylate, isobutyl methacrylate, ethylhexylacrylate, 2-ethylhexyl methacrylate, 2-ethylhexyl acrylate, isoprene,octyl acrylate, octyl methacrylate, iso-octyl acrylate, iso-octylmethacrylate, trimethylpropyl triacrylate, styrene, α-methyl styrene,chlorostyrene, 4-t-butylstyrene, 4-methyl styrene, vinyl naphthalene,vinyl toluene, chloromethyl styrene, acrylic acid, methacrylic acid,itaconic acid, crotonic acid, maleic anhydride, glycidyl methacrylate,carbodiimide methacrylate, C₁-C₁₈ alkyl crotonates, di-n-butyl maleate,di-octylmaleate, allyl methacrylate, di-allyl maleate, di-allylmalonate,methyoxybutenyl methacrylate, isobornyl methacrylate, hydroxybutenylmethacrylate, hydroxyethyl(meth)acrylate, hydroxypropyl(meth)acrylate,acetoacetoxy ethyl methacrylate, acetoacetoxy ethyl acrylate,acrylonitrile, vinyl chloride, vinylidene chloride, vinyl acetate, vinylethylene carbonate, epoxy butene, 3,4-dihydroxybutene,hydroxyethyl(meth)acrylate, methacrylamide, acrylamide, butylacrylamide, ethyl acrylamide, diacetoneacrylamide, butadiene, vinylester monomers, vinyl(meth)acrylates, isopropenyl(meth)acrylate,cycloaliphaticepoxy(meth)acrylates, ethylformamide,4-vinyl-1,3-dioxolan-2-one, 2,2-dimethyl-4 vinyl-1,3-dioxolane,3,4-di-acetoxy-1-butene, and monovinyl adipate t-butylaminoethylmethacrylate, dimethylaminoethyl methacrylate, diethylaminoethylmethacrylate, N,N-dimethylaminopropyl methacrylamide,2-t-butylaminoethyl methacrylate, N,N-dimethylaminoethyl acrylate,N-(2-methacryloyloxy-ethyl)ethylene urea, andmethacrylamido-ethylethylene urea. Further monomers are described in TheBrandon Associates, 2nd edition, 1992 Merrimack, N.H., and in Polymersand Monomers, the 1966-1997 Catalog from Polyscience, Inc., Warrington,Pa., U.S.A.

Polymerization may be carried out by a method known in the art such asbulk, solution, emulsion, or suspension polymerization. The reaction maybe free radical, cationic, anionic, zwitterionic, Ziegler-Natta, or atomtransfer radical type of polymerization. Emulsion polymerization is apreferred method of polymerization when a particularly high polymermolecular weight is desirable. A high molecular weight polymer may leadto better film quality and higher positive birefringence.

Solution film casting may be done with Ar-BES containing polymer, apolymer solution comprising a blend of Ar-BES-containing polymer withother polymers, or a copolymer of Ar-BES containing monomer with othermonomers, the latter two being advantageous because they may improvefilm quality and lower cost. Polymer solutions may further contain otheringredients such as other polymers or additives.

Depending on the particular Ar-BES structure and polymer or polymerblend composition, the Ar-BES-containing polymers may be soluble in, forexample, toluene, methyl isobutyl ketone (MIBK), methyl ethyl ketone(MEK), cyclopentanone, N,N-dimethylformamide, or mixtures thereof.Preferred solvents are toluene and MIBK.

In another example embodiment of the invention, the OASU is rod-like. Ina preferred embodiment, the rod-like structure is a mesogen. The mesogenmay be attached directly to the polymer backbone through one covalentbond (without a spacer) so the moiety has the general formula:

in the polymer backbone, wherein R¹, R², and R³ are each independentlyhydrogen atoms, alkyl groups, substituted alkyl groups, or halogens. Themesogen may also be attached directly to the polymer backbone throughtwo independent covalent bonds. The covalent bond may be a carbon-carbonor carbon-nitrogen bond. The mesogen is attached to the polymer backbonepreferably at the gravity center of the mesogen or a nearby position,but may also be attached at an end or off-center position. Themesogen-containing polymer has a positive birefringence greater than0.002 throughout the wavelength range of 400 nm<λ<800 nm without beingsubject to heat treatment, photo irradiation, or stretching. Themesogen-containing polymer film may be made by solution casting and mayform an out-of-plane anisotropic alignment upon solvent evaporation. Ina preferred embodiment, the positive birefringence is greater than0.005, greater than 0.01, greater than 0.02 or greater than 0.03throughout the wavelength range of 400 nm<λ<800 nm. Themesogen-containing polymers in the present invention are commonlyreferred to as mesogen jacketed polymers (MJPs). MJPs according to theinvention include conventional mesogen-jacketed liquid crystallinepolymers (MJLCPs) as well as polymers that are jacketed by a non-liquidcrystalline rod-like group.

The polymer film may be a homopolymer or copolymer with one or moremoieties containing a mesogen attached directly to the polymer backbonethrough at least one covalent bond. The copolymer may have a moiety withthe general structure in the polymer backbone:

wherein R¹, R², R³, R⁴, R⁵, and R⁷ are each independently hydrogenatoms, alkyl groups, substituted alkyl groups, or halogens; wherein R⁶is a hydrogen atom, alkyl group, substituted alkyl group, halogen,ester, amide, ketone, ether, cyano, phenyl, epoxy, urethane, urea, oroptically anisotropic subunit (OASU) attached directly to the backboneof the residue of an ethylenically unsaturated monomer. In oneembodiment, R⁶ is a different mesogen. The mesogen may also be attachedto a copolymer backbone by two covalent bonds.

Unlike conventional side-chain liquid crystalline polymers (LCPs) havingflexible spacers between the backbones and the mesogens, mesogen-jackedpolymers (MJPs) have no or very short spacers between the polymerbackbones and the rod-like mesogenic units. See Zhao, Y. F., et al.Macromolecules, 2006, 39, p. 948. Thus, MJPs have a strong interactionbetween the main chains and the bulky side groups. As a result, unlikethe conventional side-chain LCPs whose backbones usually take arandom-coil chain conformation, MJPs are somewhat rigid and exhibit somecharacteristics of main-chain LCPs.

It has been surprisingly found that MJPs having no spacers between thebackbones and the rod-like mesogenic side groups are capable of formingout-of-plane anisotropically aligned films without being subject toeither heat treatment or photo irradiation. An embodiment of theinvention includes preparing such films by solution casting. Uponsolvent evaporation at an ambient temperature, the resulting filmsexhibit exceptionally high positive birefringence. MJPs of the inventionare soluble in a variety of organic solvents.

Mesogens of the invention may have the general formula:R¹-(A¹-Z¹)_(m)-A²-(Z²-A³)_(n)-R²wherein,A¹, A², and A³ are independently either aromatic or cycloaliphaticrings. The rings may be all carbons or heterocyclic and may beunsubstituted or mono- or poly-substituted with halogen, cyano or nitro,or alkyl, alkoxy, or alkanoyl groups having 1 to 8 carbon atoms.Z¹, Z², and Z³ are each independently —COO—, —OOC—, —CO—, —CONH—,—NHCO—, —CH═CH—, —C≡C—, —CH═N—, —N═CH—, —N═N—, —O—, —S—, or a singlebond.R¹ and R² are each independently halogen, cyano, or nitro groups, oralkyl, alkoxy, or alkanoyl groups having 1 to 25 carbon atoms, or hasone of the meanings given for -(Z²-A³).m is 0, 1, or 2; n is 1 or 2. Preferably, m is 1 or 2; n is 1 or 2; A²is 1,4-phenylene; and the mesogen is attached to the polymer backbonethrough A². More preferably, m is 2; n is 2; A² is 1,4-phenylene; andthe mesogen is attached to the polymer backbone through A².

Representatives and illustrative examples of aromatic rings in a mesogeninclude, but are not limited to:

Representatives and illustrative examples of cycloaliphatic rings in amesogen include, but are not limited to:

Representatives and illustrative examples of mesogens that may beattached to the polymer backbone through one covalent bond include, butare not limited to:

Such mesogens may be attached to the polymer backbone via a carbon atomon a benzene ring or a nitrogen atom on a triazole. In a preferredembodiment, the mesogen is attached to the polymer backbone via a carbonatom on the center 1,4-phenylene or a nitrogen atom on the heterocyclicring.

Representatives and illustrative examples of preferred polymer moietieswith mesogens having m is 1 or 2, n is 1 or 2, A² is 1,4-phenylene, andthe mesogen is attached to the polymer backbone through A² include, butare not limited to:

Representatives and illustrative examples of preferred polymer moietieswith mesogens having m is 2, n is 2, A² is 1,4-phenylene, and themesogen is attached to the polymer backbone through A² include, but arenot limited to:

wherein R¹, R², and R³ are hydrogen atoms, alkyl groups, or halogens.

In one example embodiment of the invention, an optical film is solutioncast from polymer compositions with one or more moieties of a mesogenhaving m is 2, n is 2, A² is 1,4-phenylene, and being attached to thepolymer backbone through A² This mesogen-jacketed polymer film has anabsorption maxima between the wavelengths of about 300 nm and about 350nm and a positive birefringence greater than about 0.015 throughout 400nm<λ<800 nm. Representative and illustrative examples of such polymermoieties include, but are not limited to:

wherein R¹, R², and R³ are hydrogen atoms, alkyl groups, or halogens.

MJPs of the invention may be prepared by polymerization of a mesogenmonomer having a vinyl group attached to one of its rings, preferably anaromatic ring such as benzene. The polymerization may be carried out bya method known in the art such as bulk, solution, emulsion, orsuspension polymerization. The reaction may be free radical, cationic,anionic, zwitterionic, Ziegler-Natta, or atom transfer radical type ofpolymerization. See Zhou, Q. F., et al. Macromolecules, 1987, 20, p.233; Zhang, D., et al., Macromolecules, 1999, 32, p. 5183; Zhang, D., etal., Macromolecule, 1999, 32, p. 4494; and Chen, X., et al.,Macromolecules, 2006, 39, p. 517.

Representatives and illustrative examples of mesogen monomers withpolymerizable vinyl groups suitable for the invention include, but arenot limited to:

Representatives and illustrative examples of preferred mesogen monomerswith polymerizable vinyl groups suitable for the invention include, butare not limited to:

Polymers with these moieties have a positive birefringence greater thanabout 0.02 throughout the wavelength range of 400 nm<λ<800 nm.

MJPs of the present invention may also be prepared by copolymerizationof a mesogen monomer having one vinyl group with one or moreethylenically unsaturated monomers. Representatives and illustrativeexamples of ethylenically unsaturated monomers that may be used forcopolymerization with mesogen-containing monomers include, but are notlimited to, one or more of methyl acrylate, methyl methacrylate, ethylacrylate, ethyl methacrylate, butyl acrylate, butyl methacrylate,isobutyl acrylate, isobutyl methacrylate, ethylhexyl acrylate,2-ethylhexyl methacrylate, 2-ethylhexyl acrylate, isoprene, octylacrylate, octyl methacrylate, iso-octyl acrylate, iso-octylmethacrylate, trimethylpropyl triacrylate, styrene, α-methyl styrene,vinyl naphthalene, nitrostyrene, bromostyrene, iodostyrene,cyanostyrene, chlorostyrene, 4-t-butylstyrene, vinyl biphenyl, vinyltriphenyl, vinyl toluene, chloromethyl styrene, acrylic acid,methacrylic acid, itaconic acid, crotonic acid, maleic anhydride,glycidyl methacrylate, carbodiimide methacrylate, C₁-C₁₈ alkylcrotonates, di-n-butyl maleate, α- or β-vinyl naphthalene,di-octylmaleate, allyl methacrylate, di-allyl maleate, di-allylmalonate,methyoxybutenyl methacrylate, isobornyl methacrylate, hydroxybutenylmethacrylate, hydroxyethyl(meth)acrylate, hydroxypropyl(meth)acrylate,acetoacetoxy ethyl methacrylate, acetoacetoxy ethyl acrylate,acrylonitrile, vinyl chloride, vinylidene chloride, vinyl acetate, vinylethylene carbonate, epoxy butene, 3,4-dihydroxybutene,hydroxyethyl(meth)acrylate, methacrylamide, acrylamide, butylacrylamide, ethyl acrylamide, diacetoneacrylamide, butadiene, vinylester monomers, vinyl(meth)acrylates, isopropenyl(meth)acrylate,cycloaliphaticepoxy(meth)acrylates, ethylformamide,4-vinyl-1,3-dioxolan-2-one, 2,2-dimethyl-4 vinyl-1,3-dioxolane,3,4-di-acetoxy-1-butene, and monovinyl adipate t-butylaminoethylmethacrylate, dimethylaminoethyl methacrylate, diethylaminoethylmethacrylate, N,N-dimethylaminopropyl methacrylamide,2-t-butylaminoethyl methacrylate, N,N-dimethylaminoethyl acrylate,N-(2-methacryloyloxy-ethyl)ethylene urea, andmethacrylamido-ethylethylene urea. Further monomers are described in TheBrandon Associates, 2nd edition, 1992 Merrimack, N.H., and in Polymersand Monomers, the 1966-1997 Catalog from Polyscience, Inc., Warrington,Pa., U.S.A.

As one of skill in the art will recognize, MJP may also be prepared byfirst synthesizing a functionalized polymer and then subsequentlyreacting the polymer with a small molecule to obtain the desired mesogenstructure.

Solution film casting may be done with MJPs, a polymer solutioncomprising a blend of MJPs with other polymers, or a copolymer of MJPs,the latter two being advantageous because they may improve film qualityand lower cost. Polymer solutions may further contain other ingredientssuch as other polymers or additives. MJPs of the invention are solublein toluene, methyl isobutyl ketone (MIBK), methyl ethyl ketone (MEK),cyclopentanone, N,N-dimethylformamide or a mixture thereof depending onthe structures of the mesogens. Preferred solvents are toluene and MIBK.Optical films can be cast onto a substrate from the resulting polymersolutions by a method known in the art such as, for example, spincoating, as described above.

In another embodiment of the invention, the OASU is attached directly tothe polymer backbone through two independent covalent bonds so themoiety has the general formula:

wherein R¹, R², and R³ are each independently hydrogen atoms, alkylgroups, substituted alkyl groups, or halogens, and OASU is an opticallyanisotropic sub-unit.

Representatives and illustrative examples of such polymer moietieshaving OASU attached directly to the polymer backbone through twoindependent covalent bonds include, but are not limited to:

Representatives and illustrative examples of monomers that may be usedto prepare homopolymers or copolymers having OASU attached directly tothe polymer backbone through two independent covalent bonds include, butare not limited to:

Another example embodiment of the invention includes a method forcontrolling the birefringence of an optical compensation film byselecting a polymer that adheres to parameters that have been discoveredto enhance birefringence as disclosed herein. Birefringence of a polymerfilm with positive birefringence (positive C-plate) may be controlled bycontrolling the orientation of optically anisotropic subunits (OASUs),which are the molecular units that give a compensation film itsbirefringent properties.

In a preferred example embodiment of the invention, birefringence may becontrolled by selecting a polymer with substituents that exhibit thebuttressing effect, defined as:B=R/Dwherein B is the buttressing factor, R is the maximum dimension of anOASU in the direction perpendicular to the vector sum of the covalentbond or bonds that attach the OASU to the polymer backbone, and D is thedistance along the polymer backbone, when the polymer backbone is inextended conformation, between the attaching points of the twoneighboring OASUs. For an OASU attached to the polymer backbone by twoindependent covalent bonds, D is measured from the midpoint of the twocovalent bonds. For a polymer with some moieties that do not containOASUs, D is still the distance between attaching points of the twoclosest OASUs, even if the OASUs are not directly adjacent to each otheror if other substituents are between the OASUs along the polymer chain.The buttressing factor B of a given polymer or copolymer structure maybe calculated theoretically based on values of bond lengths and thecorresponding distances between atoms or substituents. As will beunderstood by one of ordinary skill in the art, bond lengths may becalculated by techniques such as x-ray crystallography, X-ray-absorptionfine structure, NMR spectroscopy and electron diffraction. Tablesreporting known bond lengths are known in the art and available invarious chemistry texts such as Handbook of Chemistry & Physics, 65^(th)Edition, CRC Press; Chemistry: the molecular nature of matter andchange, 4^(th) edition, 2006, The McGraw-Hill Companies, Author: MartinS. Silberberg. In one example embodiment, selection of an OASU accordingto B=R/D parameters allows control of the negative segment birefringence(Δn^(s)) of a polymer film. That polymer is then solution cast so thatit has a negative segment order parameter (O^(s)), thus resulting in apolymer film with positive birefringence (Δn).

Theoretical calculation of R and D values may be understood by referenceto an exemplary polymer, polystyrene, which is explained in FIGS. 1-2and the following example. D is the distance between the attachingpoints of two OASUs to the polymer backbone when the polymer is in theextended chain conformation, as depicted in exemplary FIG. 1 a. D is thestraight line distance between the attaching points of neighboring OASUsrather than the entire distance along the polymer backbone between theattaching points. D may be calculated by drawing a framework around anOASU-containing moiety and using known bond lengths and bond angles.

Use of the framework, bond lengths and bond angles in calculating D andR is demonstrated by reference to polystyrene. FIG. 2 a shows the pointon the polymer backbone at which the OASU of reference (the benzenering) will be attached. FIG. 2 b shows a segment of the polymer backbonein the extended chain conformation. For polystyrene, this represents twosingle carbon-carbon bonds, each having a bond length of 0.154 nm and abond angle of 109.5°. FIG. 2 c shows the distance D formed when the twoOASU-attaching points are joined by a straight line. The angles betweenthe straight line representing D and the carbon-carbon singles bonds isreadily determined [(180°−109.5°)÷2=35.2°]. Thus, the value of D may becalculated byD=(0.154×cos 35.2°)+(0.154×cos 35.2°)=0.25 nmThus, for polystyrene shown in FIG. 1 a, D is approximately 2.51 Å.Other examples of D calculations are depicted in FIGS. 1 c, 1 e, 1 g, 3a-c and 4 a-c and by Examples 23 and 24.

Turning now to FIG. 1 b, R measures the size of an OASU in the directionperpendicular to its rigid bond to the polymer backbone. The OASU isdrawn to scale in the plane of the paper according to its actual bondlengths and bond angles. R is measured by drawing lines flanking theOASU that are parallel to the covalent spacer bond and parallel to eachother and determining the distance between the two outer lines usingbond lengths and bond angles. That value is added to the van der waalsradii of the left-most and right-most atoms of the OASU. This sum willbe the value of R. This calculation is illustrated for the exemplarypolymer polystyrene in FIG. 2 d-f. FIG. 2 d shows the covalent bond froma carbon atom of the polymer backbone to the attaching atom of the OASU.In the case of polystyrene, the attaching atom is also carbon. FIG. 2 eshows the structure of benzene attached to the polymer backbone and itsknown bond lengths and bond angles. All carbon-carbon bond lengths ofthe benzene ring are 0.14 nm, all bond angles of the benzene ring are120°, and all carbon-hydrogen bond lengths of the benzene ring are 0.11nm. FIG. 2 f shows the benzene ring with parallel vertical lines drawnat intervals such that calculating the distances of segments of the OASUis possible for each interval. One skilled in the art will know how topartition each structure so that calculating R is possible based onknown bond lengths and bond angles. As shown in FIG. 2 f, bonds 1, 2, 3and 4 each have an angle of 30° with respect to the horizontal length ofthe benzene OASU, and thus each segment has a length calculated by bondlength×cos 30°, and R is the sum of these segment lengths plus the vander waals radii of the hydrogen atoms, which are each 0.12 nm. Thus, Rmay be calculated by:R=[2×cos 30°×(d _(C-C) +d _(C-H))]+[2×r _(H)]R=2×0.866×(1.4+1.1)+2×1.2R=0.67 nmAs one skilled in the art will recognize, this calculation may also beperformed as:R=(0.11 nm×cos 30°)+(0.14 nm×cos 30°)+(0.14 nm×cos 30°)+(0.11 nm×cos30°)+2×0.12 nmR=0.67 nmThus, for polystyrene shown in FIG. 1 a, R is approximately 6.7 Å. Otherexamples of R calculations are depicted in FIGS. 1 d, 1 f, 1 h, 1 i, 3d-f and 4 d-f and by Examples 23 and 24. If the OASU contains a flexibletail, such as the R group depicted at the end of the OASU in FIG. 1 i,the bond distance and bond angle of the flexible tail (R group) is notincluded in the calculation of R. Further, for Ar-BES that are styrenesubstituted with a BES at the 4-position (the position that is oppositethe attaching point of the Ar-BES to the polymer backbone), the Rcalculation will be the same as for styrene because the BES at the4-position does not contribute to the dimension (R) of the Ar-BES and isthus not included in the calculation of R. Thus, the van der waals radiiof the oxygen atoms in FIG. 1 i are the right-most and left-mostdistances to be included in the R calculation.

It will be understood by those skilled in the art that the equation forcalculating R may differ for different OASUs because it is dependent onthe bond lengths and angles of the OASU. Thus, OASUs with differentatoms or different conformations may use different equations tocalculate R, but the equation will be based on the principals describedherein.

Lastly, B is calculated by dividing R by D. Thus, for polystyrene:B=R/DB=6.7 Å÷2.51 ÅB=2.7The solution casting film of polystyrene (PS) has a birefringence around0.002 in the visible light wavelength (˜0.001-0.002 @ 633 nm).

The calculations for D and R described herein and as depicted in FIG. 1may be applied to other polymers or copolymers and thus B may becalculated for other specific OASU-containing moieties.

When the buttressing factor B is greater than about 2.5, the maximumdimension of an OASU is greater than its distance from another OASU inthe direction perpendicular to the covalent bond that attaches the OASUto the polymer backbone. These optimal parameters cause the polymerbackbone to twist into a corkscrew-like conformation such that the OASUsare oriented above and below the buttressed polymer chain, but not onthe sides of the buttressed polymer chain, to accommodate the bulkyOASUs in a sterically favorable conformation. The buttressed polymerchain is unable to unwind due to steric hindrance. The buttressingeffect also causes the polymer backbone to have an overall linear shape(i.e., viewed from a distance) over a long distance. Thus, thebuttressed polymer is rigidly fixed in the corkscrew-like conformationwith OASUs extending above and below at angles that are approximatelyperpendicular to the overall linear direction of the buttressed polymerchain, as shown in FIG. 3. The higher the perpendicularity of the OASUs,the larger the negative segment birefringence (Δn^(s)) of the polymersegment. Thus, in a preferred embodiment of the invention, thebutressing factor for an OASU is greater than about 2.5. In oneembodiment the OASU is Ar-BES and the butressing factor may be at leastabout 2.6. In a more preferred embodiment, the buttressing factor for anOASU is at least about 2.7. In one embodiment, the OASU is a disk or amesogen and the butressing factor may be at least about 2.7.

Polymer chain rigidity can be enhanced by increasing the buttressingfactor, i.e. by increasing the dimension and/or decreasing the distancebetween OASUs. Thus, the buttressing factor may be increased dependingon the desired chain rigidity, which affects the overall birefringenceof a compensation film containing the buttressed polymer. Accordingly,it may be desirable to increase the buttressing factor to any highervalue of B, such as for example 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 or anyincrements between. However, higher values of B may also be contemplateddepending on the particular application of the compensation film and itsdesired birefringence.

When the buttressed polymer backbone is in-plane (parallel to thesubstrate) and the OASUs are perpendicular to the substrate and thepolymer backbones, the perpendicularity of the OASUs (thelight-directing elements of the compensation film), allows the film tohave an index of refraction in the direction perpendicular to the film(i.e., along the optical axis of the OASUs) that is higher than theindex of refraction in the direction parallel to the film (i.e., alongthe length of the buttressed polymer's backbone. Thus, the compensationfilm satisfies the relation n_(⊥)>>n_(∥). Since the difference betweenn_(⊥) and n_(∥) is great, the birefringence Δn of the compensation filmis high.

High birefringence Δn achieved through selecting a polymer with thebuttressing effect is also desirable because it allows the thickness ofthe compensation film to be thinner since film birefringence andthickness vary inversely. The retardation value of an opticalcompensation film is defined as d×Δn, wherein d is the thickness of thefilm. Thus, a film with high birefringence can be thinner and stillachieve the desired retardation value. The retardation value of theoptical compensation film may preferably be 50 nm to 400 nm.

As described herein, the OASU may be disk-like or rod-like.Birefringence of the compensation film may be controlled by manipulatingthe birefringence of a uniaxial unit, represented as Δn^(OASU), whereinΔn^(OASU)=n_(o) ^(OASU)−n_(e) ^(OASU). Δn^(OASU) is greater than zerofor rod-like OASUs because their optical axis is in the rod direction ofOASU so n_(o) ^(OASU)>n_(e) ^(OASU). Δn^(OASU) is less than zero fordisk-like OASUs because their optical axis is perpendicular to the planeof the OASU disk so n_(o) ^(OASU)<n_(e) ^(OASU).

The order parameter of the OASU with respect to the film normaldirection (z direction) is defined as O^(OASU)=(3<cos θ>−1)/2, wherein<cos θ> is the average value of all units' cos θ value, and θ is theangle between the OASU's optical axis direction and the film's normaldirection. According to the definition, θ is in the range from 0° to90°, and O^(OASU) is thus in the range from −0.5 to 1. Therefore,O^(OASU) may be positive, negative, or zero.

A compensation film's birefringence can be expressed as a function ofthe birefringence and order parameter of a uniaxial unit:Δn=Δn^(OASU)×O^(OASU). Considering the sign of the two factors, thereare 6 combinations as listed in FIG. 4. The invention pertains topositive C plates. Thus, as shown in FIG. 4, the rod-like OASU requiresa positive order parameter (O^(OASU)>0), whereas the disk-like OASUrequires a negative order parameter (O^(OASU)<0) to satisfyΔn=Δn^(OASU)×O^(OASU)>0.

During film formation and processing, the basic unit of the polymer maybe defined as a rigid segment and the film's birefringence can beexpressed as: Δn=Δn^(S)×O^(S), wherein Δn^(S) is the birefringence ofthe segment, and O^(S) is the order parameter of the segment withrespect to the film's normal direction. The birefringence of the segmentΔn^(S) is defined as Δn^(S)=n^(S) _(o)−n^(S) _(e), wherein n^(S) _(e) isthe refractive index along the segment direction (or the main chaindirection at the segment position), and n^(S) _(e) is the averagerefractive index perpendicular to the segment direction. The orderparameter of the segment (O^(S)) with respect to the film's normaldirection is defined as O^(S)=(3<cos φ>−1)/2, wherein φ is the anglebetween the segment direction and the film's normal direction, and <cosφ> is the average value of all segments' cos φ values. For a solutioncast polymer film, O^(S) is always negative or zero. Therefore, in thecase of non-zero O^(S), the sign of Δn is determined by the sign ofΔn^(S). O^(S) preferably has a large absolute value, which requires asufficiently large segment size or a sufficiently long persistentlength.

Within segments, the OASU unit will further have an order parameter withrespect to the segment direction (units within segment), defined asO^(U-S)=(3<cos ψ>−1)/2, where ψ is the angle between the OASU opticalaxis direction and the segment direction, <cos ψ> is the average valueof all the OASU's cos ψ values within a segment. Therefore, thesegment's birefringence Δn^(S) can be expressed asΔn^(S)=Δn^(OASU)×O^(U-S). The film's birefringence Δn^(S) can thus beexpressed as: Δn=Δn^(OASU)×O^(U-S)×O^(S).

In another embodiment of the invention, birefringence may be controlledby selecting a polymer with OASUs that satisfy the equation:Δn=Δn ^(OASU) ×O ^(U-S) ×O ^(S)>0

For a rod-like OASU, Δn^(OASU)>0 and O^(S)<0. Thus, positive C-platesrequire negative O^(U-S). O^(U-S) is negative when the rods areperpendicular to the segment direction. In a preferred embodiment,rod-like OASUs are selected such that the distance between the twoattaching points of the neighboring OASUs is shorter than the length ofthe rod the so that they exhibit the buttressing effect. If the rod-likeOASUs are selected according to these parameters, then the rods can nolonger rotate freely. Instead, some conformation with rod directionparallel to the main chain direction will be forbidden due to sterichindrance. Further, the main polymer chain will be substantially rigidand linear over a long distance. All the possible conformation of therods will generate an average orientation preferably perpendicular tothe main chain and thus result in a negative O^(U-S). In thisembodiment, OASU may be attached to the main chain from any position aslong as buttressing effect is present.

For a disk-like OASU, Δn^(OASU)<0 and O^(S)<0. Thus, positive C-platesrequire positive O^(U-S). If the main chain has sufficient rigidity andthe spacer is allowed to freely rotate, O^(U-S) will be positive(according to a strict mathematic model, O^(U-S)=⅙ for this model).Considering the disk will avoid some parallel orientations due to sterichindrance, the order parameter may be more positive. In a preferredembodiment, positive O^(U-S) is achieved by selecting disk-like OASUswhen the main polymer chain is rigid and such that the distance betweenthe two neighboring attaching points of the OASUs is shorter than thelength of the rod so that they exhibit the buttressing effect.

Selecting OASUs according to the buttressing effect parameters allowsbirefringence to be manipulated because it generates and enhances thenon-zero segment parameter, O^(U-S). The buttressing effect will makesome of the OASU's conformations forbidden and thus leads to thepreferred orientation of the OASU. Especially for laterally attachedrod-like OASU, the buttressing effect is the basic reason for thenegative OASU order parameter within a segment O^(U-S). Qualitatively,the bigger buttressing factor will have a stronger buttressing effectand make O^(U-S) more negative for rod-like model and more positive fordisk-like model.

An advantage of the invention is that compensation films with highbirefringence may be obtained by a simple solution casting processwithout any other post-processing such as stretching,photopolymerization, etc. Solution casting without post-processing maysignificantly reduce the cost of film fabrication and help eliminateerrors caused by the complexity of post-processing. In certain otherembodiments, post-casting processing, such as uniaxially or biaxiallystretching or photopolymerization, may be used to further enhance thecompensation film's high positive birefringence. The order parameter ofa polymer segment, O^(S), is mainly determined by the conditions ofsolution casting, such as temperature, evaporation rate, andconcentration. Thus, the conditions of solution casting impact thebirefringence of the optical compensation film according to the equationΔn=Δn^(OASU)×O^(S) described above.

A variety of solvents may be used for solution casting the opticalcompensation films of the invention including, but not limited to,chloroform, dichloromethane, dichloroethane, benzene, chlorobenzene,xylene, N,N-dimethylformamide, N,N-dimethylacetamide,N-methyl-2-pyrrolidone, pyridine, dimethylsulfoxide, acetonitrile,cyclohexanone, methyl amyl ketone, ethyl acetate, ethylene glycolmonobutyl ether, and the like. Preferred solvents include toluene,methyl isobutyl ketone (MIBK), methyl ethyl ketone (MEK), andcyclopentanone.

Optical films may be cast onto a substrate from polymer solutions by amethod known in the art such as, for example, spin coating, spraycoating, roll coating, curtain coating, or dip coating. Substrates areknown in the art, and include TAC (triacetylcellulose), polyester,polyvinyl alcohol, cellulose ester, polycarbonate, polyacrylate,polyolefin, polyurethane, polystyrene, glass, and other materialscommonly used in an LCD device.

The solution-cast compensation film may be removed from the substrateafter drying to yield a free-standing film. The already highbirefringence of the film may optionally be further enhanced by uniaxialor biaxial stretching. The free-standing film may also be laminated ontoa substrate.

Polymer chains have a random orientation in the homogeneous solution.The conformation of polymer chains in solution is generally a randomcoil, unless the polymer backbone is rigid while molecular weight islow, which may have a rod-like shape. As shown in FIG. 5, polymer chainsin solution resemble loosely threaded-balls filled with solvent. Aftersolution casting, the balls deflate during solvent evaporation andcollapse into flatter “pancake” shapes. This process is continuous aslong as solvent evaporation continues. As a result, the order parameterof the polymer segments, O^(s), becomes more and more negative when thepolymer collapses. Polymer chain segments become aligned parallel to thesubstrate surface. However, other factors such as competition betweenthe evaporation rate of the solvent and the relaxation process of thepolymer chains determine whether or not this aligned segment orientationis maintained.

When the solution for film casting is dilute, solvent evaporationgenerally follows a free relaxation model. During the free relaxationstage, the order parameter of the polymer segments is zero. During thefree relaxation model, the system fluctuates between the collapsed ballshape and round ball shape. When the solution is dilute, the polymer'sglass transition temperature is lower than the environment temperatureso that polymer chains relax fast enough to compete with the collapse.

As solvent evaporates, the ball-shape becomes smaller and the solutionon the substrate becomes increasingly concentrated. At a certainconcentration, the glass transition temperature of the polymer chains isclose to the environmental temperature, and polymer relaxations becometoo slow to follow the solvent's evaporation or the collapsingprocedure. At this point, the polymer solution reaches the “frozenpoint” and the system follows the frozen model. At the frozen stage, theorder parameter of the polymer segment is determined by the frozenpoint, v_(f) according to the equation:O ^(s)=(v _(f)−1)/(v _(f)+2)Thus, the final order parameter of the polymer segments after solutioncasting is determined by the frozen point, v_(f). Because v_(f)'s valueis between v₀ and 1 and thus in the range 0-1, according to equationO^(S)=(v_(f)−1)/(v_(f)+2), O^(S) is always negative. Thus, thestatistical orientation of the polymer segments is always parallel tothe surfaces of the substrates. However, the value of O^(S) depends onthe value of v_(f), and O^(S) is higher when v_(f) is smaller. Thus,higher birefringence is achieved when O^(S) is large and v_(f) is small.

The value of v_(f) may be determined by many factors including, but notlimited to, evaporation rate of the solvent, environmental temperature,solubility of the polymer in the solvent and the chemical structure ofthe polymer, which affects relaxation. The evaporation rate ispreferably slow enough to ensure that the ball shape collapses but fastenough to ensure that the relaxation rate is slower at more diluteconcentrations. As is known in the art, evaporation rate may be adjustedby adjusting environmental temperature and pressure. The relaxation ratedepends on the chemical structure of polymer and the film castingtemperature. Polymers with rigid segments may freeze easily at theenvironmental temperature.

EXAMPLES Example 1 Birefringence Measurement

Polymer samples were first dissolved in suitable solvents and weresolution cast onto a piece of cleaned glass with the size of 1×1.5inches. The thickness of the polymer film was controlled in the range of15 to 20 μm by adjusting the content of solid in the solution. After thesolvent evaporated, the polymer film was peeled off the glass to obtaina piece of free-standing film. Birefringence of the free-standingpolymer films was measured by a prism coupler (Model 2010), fromMetricon Corp. at 633 nm.

Example 2 Preparation of Poly(2-vinylnaphthalene) by bulk polymerization

2-Vinylnaphthalene (2.00 g) was charged to a Schlenk tube. The tube wasstoppered, evacuated by pulling vacuum, and then filled with argon gas.The tube was evacuated and then refilled with argon four more times.While under a positive pressure of argon, the tube was immersed into anoil bath maintained at 70° C. for 24 hours. After cooling to roomtemperature, the solid plug of material was dissolved in tetrahydrofuran(THF). The solution was added in a dropwise manner into 500 mL ofrapidly stirring methanol, causing the polymer to precipitate. Theprecipitated polymer was collected by filtration and dried by pullingair through the material on a filter pad. The polymer was then dissolvedin fresh THF and reprecipitated by dropwise addition into rapidlystirring methanol. After collection by filtration and drying, theresulting polymer was found to have MW of 127,000 g/mol and a Tg of 139°C. A film cast from cyclopentanone (Cp) showed a positive birefringenceof 0.0040 at 633 nm.

Example 3 Preparation of Poly(2-vinylnaphthalene) by solutionpolymerization

2-Vinylnaphthalene (2.01 g), azo-bis(isobutyronitrile) (AIBN, 1.5 mg)and benzene (0.98 g) were charged to a 50 mL round bottom flaskcontaining a Teflon-coated magnetic stirbar. The reaction mixture wasdegassed by bubbling dry argon gas through the stirring reaction mixturefor 15 minutes. The vessel contents were then kept under a positivepressure of argon and immersed into an oil bath maintained at 60° C. for19 hours. The contents of the vessel were diluted with 25 mL of benzeneafter cooling the reaction mixture to room temperature. The resultingsolution was slowly poured into 500 mL of rapidly stirring methanol,causing the resulting polymer to precipitate. The precipitated polymerwas collected by filtration and dried by pulling air through thematerial on a filter pad. The polymer was then dissolved intetrahydrofuran and reprecipitated by dropwise addition into rapidlystirring methanol. After collection by filtration and drying, theresulting polymer was found to have MW of 251,000 g/mol and a Tg of 148°C. A film cast from cyclopentanone showed a positive birefringence of0.0073 at 633 nm.

Example 4 Preparation of Poly(2-vinylnaphthalene) by emulsionpolymerization

2-Vinylnaphthalene (2.00 g), sodium dodecyl sulfate (0.40 g), and water(18.0 g) were charged to a 125 ml round bottom flask containing aTeflon-coated magnetic stirbar. The contents of the vessel were degassedby bubbling dry argon gas through the stirring reaction mixture for 30minutes. The vessel contents were then kept under a positive pressure ofargon and immersed into an oil bath maintained at 80° C. After 30minutes at 80° C., the vessel was then charged with potassium persulfatesolution (32 mg in 1 mL of water). After the initial charge of initiatorsolution, a fresh charge of potassium persulfate solution (32 mg in 1 mLwater) was added to the reaction vessel every 2 hr. At the end of the 6hr polymerization period, the reaction mixture was poured into 250 mL ofrapidly stirring methanol. The addition of 200 mL of methylene chlorideto the resulting suspension caused the polymer to precipitate. Theprecipitated polymer was collected by filtration and dried by pullingair through the material on a glass frit filter. The polymer was thenredissolved in tetrahydrofuran and reprecipitated by dropwise additioninto rapidly stirring methanol. After collection by filtration anddrying, the resulting polymer was found to have MW of 550,000 g/mol anda Tg of 146° C. A film cast from cyclopentanone showed a positivebirefringence of 0.0062 at 633 nm.

Example 5 Preparation of Poly(1-vinylpyrene) by bulk polymerization

1-Vinylpyrene (2.0 g) was charged to a Schlenk tube. The tube wasstoppered, evacuated by pulling vacuum, and then filled with argon gas.The tube was evacuated and then refilled with argon four more times.While under a positive pressure of argon, the tube was immersed into anoil bath maintained at 100° C. for 24 hours. After cooling to roomtemperature, the solid plug of material was dissolved in tetrahydrofuran(THF). The solution was added in a dropwise manner into rapidly stirringethanol, causing the polymer to precipitate. The precipitated polymerwas collected by filtration and dried by pulling air through thematerial on a filter pad. The polymer was then dissolved in fresh THFand reprecipitated by dropwise addition into rapidly stirring ethanol.After collection by filtration and drying, the resulting polymer wasfound to have MW of 72,600 g/mol and a Tg of 254° C. A film cast fromcyclopentanone showed a positive birefringence of 0.0051 at 633 nm.

Example 6 Preparation of Poly(N-vinyl phthalimide)

To a reaction tube were charged 1.0 g of N-vinylphthalimide and 1.3 g ofa solution of benzoyl peroxide in chlorobenzene (1.0 mg/g). The reactionmixture was purged with argon, heated to 78° C., and allowed to reactovernight. After cooled down to room temperature, the solution waspoured to methanol. The resulting white precipitate was collected anddried to afford about 1 g of white powder. A film cast fromγ-butyrolactone (GBL) showed Δn=0.0154 at 633 nm (only partially solublein GBL). Another film cast from NMP showed Δn=0.0045 at 633 nm (brittlefilm).

Using the same method, two substituted poly(N-vinyl phthalimides),poly(N-vinyl-4,5-dichlorophthalimide) andpoly(N-vinyl-4-trifluoromethylphthalimide), were also prepared. However,films could not be cast due to their poor solubility.

Example 7 Preparation of Poly(N-vinyl phthalimide-co-styrene)

According to the same method as in Example 6, copolymers were preparedby charging various mole ratios of styrene (S) with either N-vinylphthalimide (VPI) or N-vinyl-4,5-dichlorophthalimide (VDCPI), Films werethen cast from NMP and their birefringence measured as listed in thetable below. It should be noted, however, the mole ratios of theresulting polymers could vary due to low yields (about 30%).

TABLE 1 Styrene/VPI or VDCPI Copolymer mole ratio used Δn at 633 nmP(S-co-VPI) 1:1 0.0035 P(S-co-VPI) 1:3 0.0031 P(S-co-VDCPI) 1:3 0.0030P(S-co-VDCPI) 7:1 0.0012

Example 8 Preparation of Poly(nitrostyrene) by Nitration of Styrene

Polystyrene (5.0 g) was stirred and dissolved in a solvent mixture ofnitrobenzene (90 g) and 1,2-dichloroethane (30 g) in a three-neckround-bottom flask equipped with a mechanical stirrer. To the stirredmixture was added a mixed acid (nitro/styrene equivalent ratio=2/1)consisting of nitric acid (8.6 g) and concentrated sulfuric acid (10.0g) dropwise in a period of 30 min. The mixture was allowed to react atroom temperature under nitrogen for a total of 22 hours. The resultingyellow mixture was poured into diluted sodium hydroxide in water andorganic layer separated, which was subsequently precipitated intomethanol to give a solid mass. The solid was dissolved inN,N-dimethylformamide (DMF) and re-precipitated into methanol. Theresulting heterogeneous mixture was stirred for two hours, filtered,washed repeatedly with methanol, and dried under vacuum to give aslightly yellowish fibrous powder. The yield was generally >95%.

Using the above method, various poly(nitrostyrenes) were prepared aslisted below. Products 1-3 were prepared using a polystyrene with weightaverage molecular weight (MW) 280,000 and Tg 100° C. (Aldrich), whileproduct 4 from one having MW 230,000 and Tg 94° C. (Aldrich).

TABLE 2 Reaction Reaction Product Time, % N Poly(nitrostyrene)Polystyrene Nitro/Styrene hours Solubility (DS) Tg, ° C. Δn @ 633 nm 1Mw 280K, Tg 3/1 22 DMF 9.63 189 +0.0138 100° C. (1.02) 2 Mw 280K, Tg 2/122 Cp, DMF 8.22 174 +0.0161 100° C. (0.87) 3 Mw 280K, Tg 2/1 6 Cp, DMF7.37 147 +0.0110 100° C. (0.78) 4 Mw 230K, Tg 2/1 22 Cp, DMF 8.69 177+0.0157 94° C. (0.92)

Product 1 was soluble in DMF but not in Cp, while the others weresoluble in Cp. Films of 2, 3, and 4 were cast respectively from their Cpsolutions by spreading the solutions on glass slides and allowing to dryat room temperature in air to form thin films (about 15-20 μm). A filmof product 1 was cast from DMF and dried under vacuum due to thehygroscopic character of DMF. % N of the polymer was determined byelemental analysis, from which the degree of substitution (DS) of thenitro group was calculated.

Example 9 The Property Relationship of Poly(Nitrostyrenes) HavingVarious Degrees of Substitution

Using the same method in Example 8, a series of poly(nitrostyrenes)having various degrees of substitution (DS) were prepared by adjustingthe nitro/styrene equivalent ratio. Their solubility and birefringencewere then determined; the results are plotted in FIG. 6. As illustratedin FIG. 6, the solubility of the poly(nitrostyrene) decreases withincreased DS. Those with DS greater than about 0.9 were only soluble inDMF, with DS between about 0.4 and about 0.9 were soluble in Cp and DMF;with DS about 0.35 were soluble in methyl isobutyl ketone (MIBK), Cp,and DMF; and with DS lower than about 0.3 were soluble in toluene, MIBK,Cp, and DMF. FIG. 6 also shows that the birefringence of thepoly(nitrostyrene) increases with increased degree of nitration.

Example 10 Preparation of Nitrostyrene Copolymer

A copolymer was prepared by nitration of poly(styrene-co-acrylonitrile)(75% styrene, MW 165K; Aldrich) using the same method as in Example 8with an equivalent ratio of nitro/styrene, 3/1. The resulting polymerhad Tg of 151° C., % N 5.84 (DS 0.62) (excluding CN group), and wassoluble in cyclopentanone (Cp). A film was cast from Cp and showed apositive birefringence of 0.0089 at 633 nm.

Example 11 Preparation of Poly(Bromostyrene) by Bromomination of Styrene

Polystyrene (5.0 g) (Mw 280,000; Aldrich) was stirred and dissolved in1,2-dichloroethane (100 g) in a three-neck-round-bottom flask equippedwith a mechanical stirrer. To the stirred mixture was added AlCl₃ (0.1g) followed by the addition of bromine (15.4 g) (Br/styrene equivalentratio, 2/1) in a period of one hour. The mixture was allowed to react atroom temperature under nitrogen for a total of 7 hours. The resultingred mixture was precipitated into methanol, filtered, and washedrepeatedly with methanol to give a slightly yellowish fibrous powder(7.2 g). The product was soluble in toluene or Cp and has a Tg of 134°C., 34% of Br (DS 0.78). A film was cast from toluene and measured tohave Δn+0.0069 @ 633 nm.

Example 12 Preparation of Bromostyrene Copolymer

A copolymer was prepared by bromination ofpoly(styrene-co-acrylonitrile) (75% styrene, MW 165K; Aldrich) using thesame method as in Example 11 with an equivalent ratio of Br/styrene,2/1. The resulting polymer had Tg of 141° C., 26% Br (DS 0.65), and wassoluble in MIBK. A film cast from MIBK showed a positive birefringenceof 0.0024 at 633 nm.

Example 13 Preparation of Poly(Bromo-Nitrostyrene) by Bromination ofPoly(Nitrostyrene)

By using the same method as in Example 11, a poly(bromo-nitrostyrene)was prepared by bromination of a poly(nitrostyrene) having DS 0.47prepared as in Example 7. In the reaction, poly(nitrostyrene) (3.0 g),AlCl₃ (0.1 g), and bromine (4.62 g) (Br/styrene 2/1) were used. Themixture was allowed to react for 5 hours to give a slightly yellowishpowder (2.5 g); Tg 139° C.; soluble in MIBK or cyclopentanone; film castfrom MIBK having Δn+0.0054 @ 633 nm.

Example 14 Preparation of Poly(Nitro-Bromostyrene) by Nitration ofPoly(Bromostyrene)

By using the same method as in Example 8, a poly(nitro-bromostyrene) wasprepared by nitration of poly(bromostyrene) prepared in Example 11. Inthe reaction, poly(nitrostyrene) (2.50 g), HNO₃ (2.15 g), and H₂SO₄(2.50 g) were used. The mixture was allowed to react for 5 hours to givea slightly yellowish powder (2.1 g); Tg 144° C.; % N 1.67; soluble incyclopentanone.

Example 15 Nitration of Poly(2-vinylnaphthalene)

A nitro-substituted polymer was prepared by nitration ofpoly(2-vinylnaphthalene) (Mw 251 K; Tg 148° C.) using the same method asin Example 8 with an equivalent ratio of nitro/styrene, 2/1. Thereaction was carried out by charging poly(2-vinylnaphthalene) (0.25 g),nitrobenzene (4.5 g), 1,2-dichloroethane (1.5 g), HNO₃ (0.29 g), andH₂SO₄ (0.34 g) to a 50 ml flask equipped with a magnetic stirrer. Themixture was allowed to react for 22 hours to give a powder (0.33 g). Theresulting polymer had Tg of 199° C. and % N 2.17 (DS 0.31) and wassoluble in cyclopentanone. A film cast from Cp showed a positivebirefringence of 0.0088 at 633 nm.

Example 16 Nitration of Poly(4-vinyl biphenyl)

Similar to Example 15, a nitro-substituted polymer was prepared bynitration of poly(4-vinyl biphenyl) (Mw 396K; Tg 150° C.) by usingpoly(4-vinyl biphenyl) (0.25 g), nitrobenzene (4.5 g),1,2-dichloroethane (1.5 g), HNO₃ (0.25 g), and H₂SO₄ (0.29 g). Theresulting polymer had Tg of 192° C. and % N 2.30 (DS 0.37) and wassoluble in cyclopentanone. A film cast from Cp showed a positivebirefringence of 0.0097 at 633 nm.

Example 17 Bromination of Poly(styrene-co-4-vinyl biphenyl)

As in Example 11, a bromo-substituted polymer was prepared bybromination of poly(styrene-co-4-vinyl biphenyl) (Mw 229K) by usingpoly(styrene-co-4-vinyl biphenyl) (0.5 g), 1,2-dichloroethane (14 g),AlCl₃ (0.04 g), and bromine (1.54 g). The resulting polymer had Tg of161° C., % Br of 35% (DS˜1) and was soluble in toluene. A film cast fromCp showed a positive birefringence of 0.0028 at 633 nm.

Example 18 Alkylation and Nitration of Polystyrene

This example illustrates an MIBK-soluble poly(nitrostyrene) with highpositive birefringence can be prepared by first reacting polystyrenewith t-butylchloride and subsequently reacting with a mixed acid.

Alkylation: Polystyrene (5.20 g) (Mw 280,000; Aldrich) was stirred anddissolved in carbon disulfide (70 g) in a three-neck round-bottom flaskequipped with a mechanical stirrer and a water condenser. To the stirredmixture was added AlCl₃ (0.01 g), followed by the addition of t-butylchloride (2.31 g) (t-butyl/styrene equivalent ratio, 1/2). The mixturewas allowed to reflux under nitrogen for 2 hours and then allowed tocool to room temperature. The resulting mixture was precipitated intomethanol, filtered, washed repeatedly with methanol, and dried undervacuum to give a fibrous powder (6.32 g). The product was soluble inMIBK and has a Tg of 117° C. A film was cast from MIBK and measured tohave Δn=0.0027 @ 633 nm.

Nitration: The above product after alkylation (1.5 g) was stirred anddissolved in nitrobenzene (25 g). To the mixture was added a mixed acidof HNO₃ (2.6 g) and H₂SO₄ (2.6 g) dropwise in a period of 30 minutes.The mixture was allowed to react at room temperature for 24 hours. Afterthe reaction, the yellow mixture was washed with dilute NaOH in water.The organic layer was separated and precipitated into methanol,filtered, and then dissolved in DMF. The resulting polymer solution wasre-precipitated into methanol, filtered, washed repeatedly withmethanol, and dried under vacuum to give a yellowish fibrous powder(1.77 g). The product was soluble in MIBK and has a Tg of 171° C. A filmwas cast from MIBK and measured to have Δn 0.0086 @ 633 nm.

Example 19 Nitration of Poly(4-methylstyrene)

This example illustrates that the nitro group can be incorporated ontopositions other than the para position of styrene and still enhance thebirefringence of the polymer film.

Poly(4-methylstyrene) (5.0 g; available from Scientific PolymerProducts, Inc.; MW 100K) was stirred and dissolved in (100 g) in athree-neck round-bottom flask equipped with a mechanical stirrer. To thestirred mixture was added a mixed acid (nitro/styrene equivalentratio=2/1) consisting of nitric acid (8.6 g) and concentrated sulfuricacid (17.2 g) dropwise in a period of 30 min. The mixture was allowed toreact at room temperature under nitrogen for a total of 20 hours. Theresulting yellow mixture was poured into diluted sodium hydroxide inwater and organic layer separated, which was subsequently precipitatedinto methanol to give a solid mass. The solid was dissolved inN,N-dimethylformamide (DMF) and re-precipitated into methanol. Theresulting heterogeneous mixture was stirred for two hours, filtered,washed repeatedly with methanol, and dried under vacuum to give aslightly yellowish fibrous powder. The product (about 95% yield) wassoluble in cyclopentanone but not in MIBK or toluene. A film cast fromcyclopentanone showed a positive birefringence of 0.0060 at 633 nm. (Thestarting material, poly(4-methylstyrene), was determined to haveΔn=0.0017 at 633 nm.)

Comparative Example 20 Substituted Polystyrenes Having Low PositiveBirefringence Values

This example shows that, in contrast to the BES-substituted polystyrenesof the present invention, the following substituted polystyrenes havelow positive birefringence values.

TABLE 3 Birefringence at 633 nm (solvent used for film Molecular Weightcasting) Polystyrene 280K 0.0012 (Toluene); 0.0020 (Cp)Poly(4-methylstyrene) 100K 0.0017 (Toluene) Poly(4-methoxystyrene) 400K0.0028 (Toluene); 0.0024 (Cp) Poly(4-chlorostyrene) N/A 0.0020(Toluene); 0.0022 (Cp)

Example 21 Preparation of Poly(4-vinylbiphenyl) by bulk polymerization

4-Vinylbiphenyl (1.38 g) was charged to a Schlenk tube. The tube wasstoppered, evacuated by pulling vacuum, and then filled with argon gas.The tube was evacuated and then refilled with argon four more times.While under a positive pressure of argon, the tube was immersed into anoil bath maintained at 130° C. for 1.5 hours. After cooling to roomtemperature, the solid plug of material was dissolved in tetrahydrofuran(THF). The solution was added in a dropwise manner into 500 mL ofrapidly stirring methanol, causing the polymer to precipitate. Theprecipitated polymer was collected by filtration and dried by pullingair through the material on a filter pad. The polymer was then dissolvedin fresh THF and reprecipitated by dropwise addition into rapidlystirring methanol. After collection by filtration and drying, theresulting polymer was found to have MW of 396,000 g/mol and a Tg of 150°C. A film cast from cyclopentanone showed a positive birefringence of0.0071 at 633 nm.

Example 22 Preparation of Poly(4-cyanostyrene) by solutionpolymerization

4-Cyanostyrene (1.65 g), azo-bis-isobutyronitrile (AIBN, 11 mg) andN,N-dimethylacetamide (DMAc, 1.65 g) were charged to a 50 mL roundbottom flask containing a Teflon-coated magnetic stirbar. The reactionmixture was degassed by bubbling dry argon gas through the stirringreaction mixture for 15 minutes. The vessel contents were then keptunder a positive pressure of argon and immersed into an oil bathmaintained at 60° C. for 2.5 hours. The contents of the vessel werediluted with 25 mL of DMAc after cooling the reaction mixture to roomtemperature. The resulting solution was slowly poured into 500 mL ofrapidly stirring methanol, causing the resulting polymer to precipitate.The precipitated polymer was collected by filtration and dried bypulling air through the material on a filter pad. The polymer was thendissolved in fresh DMAc and reprecipitated by dropwise addition intorapidly stirring methanol. After collection by filtration and drying,the resulting polymer was found to have MW of 842,000 g/mol and a Tg of184° C. A film cast from cyclopentanone showed a positive birefringenceof 0.0103 at 633 nm.

Comparative Example 23 Vinyl Polymers Having High Tg and Low PositiveBirefringence

Polymers 1-5 were synthesized by free-radical solution polymerizationand their Tg and birefringence values determined as listed in thefollowing table:

TABLE 4 Polymer Structure Tg (° C.) Δn at 633 nm 1 Poly(2-phenylaminocarbonylstyrene)

200 0.0032 2 Poly(4-cyanophenyl methacrylate)

161 0.0009 3 Poly(methylcarboxyphenyl methacrylamide)

211 0.0011 4 Poly(isobornyl methacrylate)

191 0.0006 5 Poly(phenyl methacrylamide)

160 0.0020As shown in Table 4, Tg and positive birefringence are not directlyproportional.

Example 24 Synthesis of Various Mesogen-Jacketed Polymers

The following mesogen-jacketed polymers were made by charging the vinylmesogen monomers, benzoyl peroxide (BPO, 0.1-0.3% mole of monomers) andtoluene or chlorobenzene to a polymerization tube containing aTeflon-coated magnetic stirbar. The reaction mixture was degassed bybubbling argon through for 15 minutes. The tube was then sealed andimmersed into an oil bath maintained at 80° C. for one day. Aftercooling the reaction mixture to room temperature, it was slowly pouredinto rapidly stirring methanol, causing the resulting polymer toprecipitate. The precipitated polymer was collected by filtration anddried in a vacuum oven.

1. Poly[2,5-bis(p-alkoxyphenyl)styrene]

wherein R₁=—OCH₂CH(CH₃)CH₂CH₃, R₂=—OCH₂CH₂OCH₃ Δn=0.0082

2. Poly{2,5-bis[5-(4-substitutedphenyl)-1,3,4-oxadiazole]styrene}

wherein R=—OC(CH₃)₃, —OC₆H₁₃, —OC₈H₁₇, —OC₁₀H₂₁, —OC₁₂H₂₅

R Mn (×10⁻⁴) Tg (° C.) Δn —OC(CH₃)₃ 16 201 0.0184 —OC₆H₁₃ 16 — 0.0355—OC₈H₁₇ 23 141 0.0362 —OC₁₀H₂₁ 23 — 0.0295 —OC₁₂H₂₅ — 150 0.0229

3. Poly{3,5-bis[5-(4-ter-butylphenyl)-1,3,4-oxadiazole]styrene}

4. Poly{4-[5-(4-substitutedphenyl)-1,3,4-oxadiazole]styrene} andPoly{2-[5-(4-substitutedphenyl)-1,3,4-oxadiazole]styrene}

wherein R=phenyl, Δn=0.009

wherein R=—OC₈H₁₇, Tg=130° C., Δn=0.009; R=phenyl, Δn=0.009

5. Poly{2-(naphthalen-2-yl)-5-(prop-1-en-2-yl)-1,3,4-oxadiazole}

6. Triazole Based Mesogen-Jacketed Polymers

wherein

-   -   Ar₁=4-(dodecyloxy)phenyl, Ar2=t-butyl; Δn=0.0045    -   Ar₁=4-(octyloxy)phenyl, Ar2=4-(octyloxy)biphenyl; Δn=0.011    -   Ar₁=4-(dodecyloxy)phenyl, Ar2=biphenyl; Δn=0.010    -   Ar₁=4-(dodecyloxy)biphenyl, Ar2=biphenyl; Δn=0.024

The following mesogen-jacketed polymers are suitable for the practice ofthis invention as well.

7. Poly-2,5-bis[(4-substitutedbenzoyl)oxy]styrene

8. Poly-2,5-bis(4-substitutedbenzamido)styrene

9. Poly-2,5-bis[(4-substitutedphenyloxy)carbonyl]styrene

10. Poly{2,5-bis[(4-methoxyphenyloxy)carbonyl]styrene-b-styrene}

wherein m=2000, n=600, Tg: 100° C. and 120° C., Δn=0.010

Example 25 Poly(2-vinyl naphthalene)

The buttressing factor B was calculated for a naphthalene ring OASU.Poly(2-vinyl naphthalene) has the same backbone structure as polystyrene(PS) and thus D is calculated the same as for polystyrene, yieldingD=0.25 nm. The attaching atom of the OASU is the 2-position carbon atomon the naphthalene ring. All of the carbon-carbon bond lengths of thenaphthalene ring are 0.14 nm, all bond angles of the naphthalene ringare 120°, and all carbon-hydrogen bond lengths of the naphthalene ringare 0.11 nm as shown in FIG. 7 b. When parallel lines are drawn throughthe center of each atom of the OASU, as shown in FIG. 7 c, of the bondangles of bonds 1, 2, 3, 4 and 5 with respect to the horizontal lengthof the OASU are all 30°. The left-most and right-most atoms are hydrogenatoms and have van der waals radii of 0.12 nm. Thus, R is calculated by:R=(0.11 nm*cos 30°)+(0.14 nm*cos 30°)+(0.14 nm*cos 30°)+(0.14 nm*cos30°+(0.11 nm*cos 30°)+0.12 nm+0.12 nm=0.79 nm.

The buttressing factor is calculated by B=R/D:B=R/D=0.79 nm/0.25 nm=3.2.

This R/D value is bigger than PS and thus lead to a stronger buttressingeffect and higher positive birefringence as compared to PS. Also, thenaphthalene OASU has a bigger Δn^(OASU) than PS, which enhanced theoverall Δn. The solution cast poly(2-vinyl naphthalene) film showed apositive birefringence of 0.0073 at 633 nm.

Example 26 polyvinylpyrene (PVPr)

The buttressing factor B was calculated for a pyrene OASU. PVPr has thesame backbone structure as polystyrene (PS) and thus D is calculated thesame as for PS, yielding D=0.25 nm. The attaching atom of the OASU is acarbon atom on the pyrene ring. All of the carbon-carbon bond lengths ofthe pyrene ring are 0.14 nm, all bond angles of the pyrene ring are120°, and all carbon-hydrogen bond lengths of the pyrene ring are 0.11nm shown in FIG. 8 b. When parallel lines are drawn through the centerof each atom of the OASU, the bond angles of bonds 1, 2, 3, 4, 5, 6 and7 with respect to the horizontal length of the OASU as shown in FIG. 8 care all 30°. The left-most and right-most atoms are hydrogen atoms andhave van der waals radii of 0.12 nm. Thus, R is calculated by:R=(0.11 nm×cos 30°)+(0.14 nm×cos 30°)+(0.14 nm×cos 30°)+(0.14 nm×cos30°)+(0.14 nm×cos 30°)+(0.14 nm×cos 30°)+(0.11 nm×cos 30°)+0.12 nm+0.12nm=1.04 nm

The buttressing factor is calculated by B=R/D:B=R/D=1.04/0.25=4.1

This R/D value is bigger than PS and thus lead to stronger buttressingeffect and higher positive birefringence as compared to PS. Also, thepyrene OASU has a bigger Δn^(OASU) than PS, which enhanced the final Δn.The solution cast PVPr film showed a positive birefringence of 0.0051 at633 nm.

Example 27 Substituted Polystyrene

A BES at the 4-position of benzene ring enhanced the birefringence ofpolystyrene without changing its buttressing effect. Polystyrene wassubstituted at the 4-position with the following BES: Cl, Br, I, CN,NO₂, and phenyl. Each BES-substituted polystyrene had the samebuttressing factor (R/D value) as unsubstituted polystyrene butexhibited enhanced birefringence. Using the highly polarizable and polarNO₂ group enhanced the birefringence of BES-substituted polystyrene toas high as 0.0209.

Example 28 Mesogen-Jacketed Polymer

The buttressing factor was calculated for the mesogen OASU depictedabove. The mesogen OASU has the same backbone structure as polystyrene(PS) and thus D is calculated the same as for PS, yielding 0.25 nm. Theattaching atom of the OASU is a carbon atom on the middle benzene ringof the mesogen OASU. All of the carbon-carbon bond lengths of thebenzene rings are 0.14 nm, all bond angles of the benzene rings are120°, the carbon-carbon bond between two benzene rings is 0.15 nm, andall carbon-oxygen bond lengths of the mesogen are 0.14 nm shown in FIG.9. The length between carbon 1 and carbon 4 of the benzene rings(depicted as lines 2, 4 and 6 in FIG. 9) is 0.28 nm. Although the threebenzene rings don't lie in the same plane because the OASU may twist onits axis, the benzene rings maintain a linear alignment. When the twooxygen atoms are connected by a straight line as shown in FIG. 9, the1-, 4-carbon atoms of all three benzene rings lie on that line. Whenparallel lines are drawn through the center of each atom of the OASU,the bond angles of bonds 1, 3, 5 and 7 and lines 2, 4 and 6 with respectto the horizontal length of the OASU as shown in FIG. 9 are all 30°. Theleft-most and right-most atoms are oxygen atoms and have van der waalsradii of 0.15 nm. The R₁ and R₂ alkyl groups are not included in thecalculation of R because their bonds are flexible. Thus, R is calculatedby:R=(0.14 nm×cos 30°)+(0.28 nm×cos 30°)+(0.15 nm×cos 30°)+(0.28 nm×cos30°)+(0.15 nm×cos 30°)+(0.28 nm×cos 30°)+(0.14 nm×cos 30°)+0.15 nm+0.15nm=1.53 nm

The buttressing factor is calculated by B=R/D:B=R/D=1.53/0.25=6.1

This R/D value is bigger than PS and thus lead to stronger buttressingeffect and higher positive birefringence as compared to PS. Also, thismesogen OASU has a bigger Δn^(OASU) than PS, which enhanced the finalΔn. The solution cast this mesogen jacket polymer film (with R₁ as—OCH₂CH₂OCH₃, and R₂ as —CH₂CH(CH₃)CH₂CH₃) showed a positivebirefringence of 0.0082 at 633 nm.

Example 29 Preparation of Poly(N-vinyl-4-tert-butylphthalimide) bysolution polymerization

N-Vinyl-4-tert-butylphthalimide (2.0 g), chlorobenzene (6.0 g), andbenzoyl peroxide (2.1 mg) were charged to a Schlenk tube containing aTeflon-coated magnetic stirbar. The tube was stoppered and degassedthrough the sidearm by three freeze-pump-thaw cycles. While under apositive pressure of argon, the reaction tube was immersed into an oilbath maintained at 85° C. for 3 hours with constant stirring. Aftercooling to room temperature, the resulting viscous solution was dilutedwith 10 mL of tetrahydrofuran (THF) and added in a dropwise manner into500 mL of rapidly stirring methanol, causing the polymer to precipitate.The precipitated polymer was collected by filtration and dried bypulling air through the material on a filter pad. The polymer wasreprecipitated twice more from fresh THF solution by dropwise additioninto methanol. After collection by filtration and drying, the resultingpolymer was found to be soluble in MIBK and toluene and had a Tg of 215°C. and a weight average molecular weight (Mw): 643, 000. A film castfrom toluene showed a positive birefringence of 0.0094 at 633 nm.

Example 30 UV Spectra of Various Mesogen-Jacketed Polymers

The following table is a collection of the absorption maximum (λmax) andthe birefringence (Δn) measured at the wavelength of 633 nm for eachpolymer. The first five polymers (PC6, PC8, PC10, PC12, PCt) arepoly{2,5-bis[(4-alkyloxyphenyl)-1,3,4-oxadiazole]styrene} with thefollowing chemical structure:

wherein R₁ and R₂ are defined in Table 5 below.

Sample XCt is poly{3,5-bis[(4-ter-butylphenyl)-1,3,4-oxadiazole]styrene}with the following chemical structure:

TABLE 5 Sample R₁ = R₂ λmax, nm Δn(633) PC6 —OC₆H₁₃ 319.8 0.0355 PC8—OC₈H₁₇ 319.1 0.0362 PC10 —OC₁₀H₂₁ 315.7 0.0295 PC12 —OC₁₂H₂₅ 319.80.0229 PCt —C(CH₃)₃ 309.6 0.0184 XCt N/A 281.6 0.0117

UV-Visible Spectrophotomer (UV-2450) from Shimadzu (Japan) was used toobtain UV spectrum of the above polymers, shown in Figure X. λ_(max) isthe wavelength of the absorption maximum. Δn₍₆₃₃₎ was measured withPrism Coupler (Model 2010) from Mitricon Corp.

The present invention is not to be limited in scope by the specificembodiments described herein which are intended as single illustrationsof individual aspects of the invention, and functionally equivalentmethods and components are within the scope of the invention. Indeed,various modifications of the invention, in addition to those shown anddescribed herein will become apparent to those skilled in the art fromthe foregoing description and accompanying drawings. Such modificationsare intended to fall within the scope of the claims. Variouspublications are cited herein, the contents of which are herebyincorporated, by reference, in their entireties.

1. A method for controlling positive birefringence in a compensation film for liquid crystal display comprising: (a) determining a value of R/D for a polymer, wherein the polymer comprises a polymer segment having a polymer backbone, a light-stable optically anisotropic sub-unit (OASU) attached directly to the polymer backbone via at least one covalent bond, wherein R represents the maximum dimension of the OASU in the direction perpendicular to the direction of the vector sum of the at least one covalent bond and D represents the distance along the polymer backbone between the attaching points of two neighboring OASUs, and wherein OASU affects the rigidity and long-range linear corkscrew shape of the polymer backbone such that the average orientation of the OASU is perpendicular to the polymer backbone, and the higher the perpendicularity of the OASUs, the larger the value of the negative segment birefringence of the polymer segment; (b) selecting the polymer as having a controlled negative segment birefringence if said value of R/D>about 2.7; and (c) processing the polymer having a controlled negative segment birefringence (Δn^(s)) by solution casting onto a substrate, followed by uniaxial stretching or biaxial stretching or a combination thereof, wherein the polymer has a negative segment order parameter (O^(s)) and the polymer film has a positive birefringence (Δn) that satisfies the relation Δn=Δn^(s)×O^(s)>0.
 2. The method of claim 1 wherein the polymer comprises a moiety of

wherein R₁, R₂, and R₃ are each independently hydrogen atoms, alkyl groups, substituted alkyl groups, or halogens, wherein OASU is a disk-like group or a mesogen, and wherein OASU is attached to the polymer backbone through a single covalent bond.
 3. The method of claim 1 wherein the polymer comprises a moiety of

wherein R₁ and R₃ are each independently hydrogen atoms, alkyl groups, substituted alkyl groups, or halogens, wherein OASU is a disk-like group or a mesogen, and wherein OASU is attached to the polymer backbone through two independent covalent bonds.
 4. The method of claim 3, wherein the polymer moiety is selected from the group consisting of the following structure:


5. The method of claim 1 wherein the compensation film is capable of forming an out-of-plane anisotropic alignment upon solvent evaporation without being subject to heat treatment, photo irradiation, or stretching and has a positive birefringence greater than 0.002 throughout the wavelength range of 400 nm<λ<800 nm.
 6. The method of claim 5 wherein the compensation film is removed from the substrate upon drying to yield a free-standing film.
 7. The method of claim 6, wherein the free-standing film is uniaxially or biaxially stretched.
 8. The method of claim 6, wherein the free-standing film is attached to a substrate by means of lamination.
 9. The method of claim 7, wherein the free-standing film is attached to a substrate by lamination.
 10. The method of claim 1 wherein the compensation film is capable of forming an out-of-plane anisotropic alignment upon solvent evaporation without being subject to heat treatment, photo irradiation, or stretching and has a positive birefringence greater than 0.005 throughout the wavelength range of 400 nm<λ<800 nm.
 11. The method of claim 10 wherein the compensation film is removed from the substrate upon drying to yield a free-standing film.
 12. The method of claim 11, wherein the free-standing film is uniaxially or biaxially stretched.
 13. The method of claim 11, wherein the free-standing film is attached to a substrate by means of lamination.
 14. The method of claim 12, wherein the free-standing film is attached to a substrate by lamination.
 15. The method of claim 1 wherein the compensation film is capable of forming an out-of-plane anisotropic alignment upon solvent evaporation without being subject to heat treatment, photo irradiation, or stretching and has a positive birefringence greater than 0.01 throughout the wavelength range of 400 nm<λ<800 nm.
 16. The method of claim 15 wherein the compensation film is removed from the substrate upon drying to yield a free-standing film.
 17. The method of claim 16, wherein the free-standing film is uniaxially or biaxially stretched.
 18. The method of claim 16, wherein the free-standing film is attached to a substrate by means of lamination.
 19. The method of claim 17, wherein the free-standing film is attached to a substrate by lamination.
 20. The method of claim 1 wherein the compensation film is capable of forming an out-of-plane anisotropic alignment upon solvent evaporation without being subject to heat treatment, photo irradiation, or stretching and has a positive birefringence greater than 0.02 throughout the wavelength range of 400 nm<λ<800 nm.
 21. The method of claim 20 wherein the compensation film is removed from the substrate upon drying to yield a free-standing film.
 22. The method of claim 21, wherein the free-standing film is uniaxially or biaxially stretched.
 23. The method of claim 21, wherein the free-standing film is attached to a substrate by means of lamination.
 24. The method of claim 22, wherein the free-standing film is attached to a substrate by lamination.
 25. The method of claim 1 wherein the compensation film is capable of forming an out-of-plane anisotropic alignment upon solvent evaporation without being subject to heat treatment, photo irradiation, or stretching and has a positive birefringence greater than 0.03 throughout the wavelength range of 400 nm<λ<800 nm.
 26. The method of claim 25 wherein the compensation film is removed from the substrate upon drying to yield a free-standing film.
 27. The method of claim 26, wherein the free-standing film is uniaxially or biaxially stretched.
 28. The method of claim 26, wherein the free-standing film is attached to a substrate by means of lamination.
 29. The method of claim 27, wherein the free-standing film is attached to a substrate by lamination.
 30. The method of claim 1 wherein polymer composition is soluble in a solvent selected from the group consisting of toluene, methyl isobutyl ketone, cyclopentanone, and a mixture thereof.
 31. The method of claim 1 wherein polymer composition is soluble in toluene or methyl isobutyl ketone.
 32. The method of claim 1 wherein the compensation film is used in a liquid crystal display device.
 33. The method of claim 32 wherein the liquid crystal display device is an in-plane switching liquid crystal display device.
 34. The method of claim 32 wherein the liquid crystal display device is used as a screen for a television or computer.
 35. The method of claim 1 wherein the OASU is a disk, wherein the disk comprises a fused ring structure having at least two rings.
 36. The method of claim 35 wherein the polymer is a homopolymer.
 37. The method of claim 36 wherein the fused ring structure is an aromatic imide or lactam.
 38. The method of claim 36 wherein the disk is selected from the group consisting of naphthalene, anthracene, pyrene, and phthalimide.
 39. The method of claim 36 wherein the disk is selected from the group consisting of the following structures, wherein the disk is attached to the polymer backbone via a carbon atom on a benzene ring or a nitrogen atom on an imide or lactam group:


40. The method of claim 36 wherein the homopolymer is a reaction product of a monomer selected from the group consisting of:


41. The method of claim 36 wherein the polymer is poly(2-vinylnaphthalene).
 42. The method of claim 41 wherein the poly(2-vinylnaphthalene) is prepared by emulsion polymerization and has an average molecular weight of greater than 300,000 g/mol.
 43. The method of claim 36 wherein the polymer is poly(N-vinyl-4-tert-butylphthalimide).
 44. The method of claim 36 wherein the compensation film is capable of forming an out-of-plane anisotropic alignment upon solvent evaporation without being subject to heat treatment, photo irradiation, or stretching and has a positive birefringence greater than 0.002 throughout the wavelength range of 300 nm<λ<800 nm.
 45. The method of claim 36 wherein the compensation film is capable of forming an out-of-plane anisotropic alignment upon solvent evaporation without being subject to heat treatment, photo irradiation, or stretching and has a positive birefringence greater than 0.005 throughout the wavelength range of 400 nm<λ<800 nm.
 46. The method of claim 36 wherein the polymer is soluble in a solvent selected from the group consisting of toluene, methyl isobutyl ketone, cyclopentanone, and a mixture thereof.
 47. The method of claim 36 wherein the polymer is soluble in toluene or methyl isobutyl ketone.
 48. The method of claim 36 wherein the compensation film is used in a liquid crystal display device.
 49. The method of claim 48 wherein the liquid crystal display device is an in-plane switching liquid crystal display device.
 50. The method of claim 48 wherein the liquid crystal display device is used as a screen for a television or computer.
 51. The method of claim 1 wherein the OASU is a rod-like mesogen.
 52. The method of claim 51 wherein the mesogen has the structure R¹-(A¹-Z¹)_(m)-A²-(Z²-A³)_(n)-R² wherein A¹, A², and A³ are each independently aromatic or cycloaliphatic rings, wherein the rings are all-carbon or heterocyclic, wherein the rings are unsubstituted, mono- or poly-substituted with halogen, cyano or nitro groups or alkyl, alkoxy, or alkanoyl groups having 1 to 8 carbon atoms; wherein Z¹, and Z² are each independently —COO—, —OOC—, —CO—, —CONH—, —NHCO—, —CH═CH—, —C≡C—, —CH═N—, —N═CH—, —N═N—, —O—, —S—, or a single bond; wherein R¹ and R² are each independently halogen, cyano, nitro, or alkyl, alkoxy, or alkanoyl groups having 1 to 25 carbon atoms, or are (Z²-A³) as defined above; wherein m is 0, 1, or 2; and wherein n is 1 or
 2. 53. The method of claim 52 wherein m is 1 or 2, n is 1 or 2, A² is 1,4-phenylene, and the mesogen is attached to the polymer backbone through A².
 54. The method of claim 52 wherein m is 2, n is 2, A² is 1,4-phenylene, and the mesogen is attached to the polymer backbone through A².
 55. The method of claim 51 wherein the polymer is a homopolymer.
 56. The method of claim 55 wherein the mesogen is selected from the group consisting of the following structures, wherein the mesogen is attached to the polymer backbone via a carbon atom on a benzene ring


57. The method of claim 55 wherein the mesogen is selected from the group consisting of the following structures, wherein the mesogen is attached to the polymer backbone via a carbon atom on the center 1,4-phenylene:


58. The method of claim 55 wherein the polymer is a reaction product of a monomer selected from the group consisting of:

wherein the polymer has a positive birefringence greater than about 0.02 throughout the wavelength range of 400 nm<λ<800 nm.
 59. The method of claim 55 wherein the polymer is a reaction product of a monomer selected from the group consisting of:


60. The method of claim 55 wherein a moiety in the polymer backbone is selected from the group consisting of the following structures, wherein R¹, R² and R³ are each independently hydrogen, alkyl group, substituted alkyl group or halogen:

wherein the polymer has a positive birefringence greater than about 0.01 throughout the wavelength range of 400 nm<λ<800 nm.
 61. The method of claim 55 wherein a moiety in the polymer backbone is selected from the group consisting of the following structures, wherein R¹, R² and R³ are each independently hydrogen, alkyl group, substituted alkyl group or halogen:

wherein the polymer has a positive birefringence greater than about 0.02 throughout the wavelength range of 400 nm<λ<800 nm.
 62. The method of claim 55 wherein the compensation film is capable of forming an out-of-plane anisotropic alignment upon solvent evaporation without being subject to heat treatment, photo irradiation, or stretching and has a positive birefringence greater than 0.002 throughout the wavelength range of 400 nm<λ<800 nm.
 63. The method of claim 62 wherein the compensation film is removed from the substrate upon drying to yield a free-standing film.
 64. The method of claim 63, wherein the free-standing film is uniaxially or biaxially stretched.
 65. The method of claim 63, wherein the free-standing film is attached to a substrate by means of lamination.
 66. The method of claim 64, wherein the free-standing film is attached to a substrate by lamination.
 67. The method of claim 55 wherein the compensation film is capable of forming an out-of-plane anisotropic alignment upon solvent evaporation without being subject to heat treatment, photo irradiation, or stretching and has a positive birefringence greater than 0.005 throughout the wavelength range of 400 nm<λ<800 nm.
 68. The method of claim 67 wherein the compensation film is removed from the substrate upon drying to yield a free-standing film.
 69. The method of claim 68, wherein the free-standing film is uniaxially or biaxially stretched.
 70. The method of claim 68, wherein the free-standing film is attached to a substrate by means of lamination.
 71. The method of claim 69, wherein the free-standing film is attached to a substrate by lamination.
 72. The method of claim 55 wherein the compensation film is capable of forming an out-of-plane anisotropic alignment upon solvent evaporation without being subject to heat treatment, photo irradiation, or stretching and has a positive birefringence greater than 0.01 throughout the wavelength range of 400 nm<λ<800 nm.
 73. The method of claim 72 wherein the compensation film is removed from the substrate upon drying to yield a free-standing film.
 74. The method of claim 73, wherein the free-standing film is uniaxially or biaxially stretched.
 75. The method of claim 73, wherein the free-standing film is attached to a substrate by means of lamination.
 76. The method of claim 74, wherein the free-standing film is attached to a substrate by lamination.
 77. The method of claim 55 wherein the compensation film is capable of forming an out-of-plane anisotropic alignment upon solvent evaporation without being subject to heat treatment, photo irradiation, or stretching and has a positive birefringence greater than 0.02 throughout the wavelength range of 400 nm<λ<800 nm.
 78. The method of claim 77 wherein the compensation film is removed from the substrate upon drying to yield a free-standing film.
 79. The method of claim 78, wherein the free-standing film is uniaxially or biaxially stretched.
 80. The method of claim 78, wherein the free-standing film is attached to a substrate by means of lamination.
 81. The method of claim 79, wherein the free-standing film is attached to a substrate by lamination.
 82. The method of claim 55 wherein the compensation film is capable of forming an out-of-plane anisotropic alignment upon solvent evaporation without being subject to heat treatment, photo irradiation, or stretching and has a positive birefringence greater than 0.03 throughout the wavelength range of 400 nm<λ<800 nm.
 83. The method of claim 82 wherein the compensation film is removed from the substrate upon drying to yield a free-standing film.
 84. The method of claim 83, wherein the free-standing film is uniaxially or biaxially stretched.
 85. The method of claim 83, wherein the free-standing film is attached to a substrate by means of lamination.
 86. The method of claim 84, wherein the free-standing film is attached to a substrate by lamination.
 87. The method of claim 55 wherein polymer composition is soluble in a solvent selected from the group consisting of toluene, methyl isobutyl ketone, cyclopentanone, and a mixture thereof.
 88. The method of claim 55 wherein polymer composition is soluble in toluene or methyl isobutyl ketone.
 89. The method of claim 55 wherein the compensation film is used in a liquid crystal display device.
 90. The method of claim 89 wherein the liquid crystal display device is an in-plane switching liquid crystal display device.
 91. The method of claim 89 wherein the liquid crystal display device is used as a screen for a television or computer.
 92. A method for controlling positive birefringence in a compensation film for liquid crystal display comprising: (a) determining a value of R/D for a copolymer, wherein the copolymer comprises a moiety:

wherein R¹, R², R³, R⁴, R⁵, and R⁷ are each independently hydrogen atoms, alkyl groups, substituted alkyl groups, or halogens; wherein R⁶ is a group wherein R⁶ is a hydrogen atom, alkyl, substituted alkyl, halogen, ester, amide, ketone, ether, cyano, phenyl, epoxy, urethane, urea or optically anisotropic subunit (OASU) attached directly to the backbone of a residue of an ethylenically unsaturated monomer; wherein disk is an optically anisotropic subunit (OASU) having a fused ring structure comprising at least two rings, wherein the disk is attached directly to the copolymer backbone via at least one covalent bond, wherein R represents the maximum dimension of the disk in the direction perpendicular to the direction of the vector sum of the at least one covalent bond and D represents the distance along the copolymer backbone between the attaching points of disk and R⁶, wherein said disk affects the rigidity and long-range linear corkscrew shape of the copolymer backbone such that the average orientation of the disk is perpendicular to the copolymer backbone, and the higher the perpendicularity of the disks, the larger the value of the negative segment birefringence of the polymer segment; (b) selecting said copolymer as having a controlled negative segment birefringence if said value of R/D>about 2.7; and (c) processing the copolymer having a controlled negative segment birefringence (Δn^(s)) by solution casting onto a substrate, followed by uniaxial stretching or biaxial stretching or a combination thereof, wherein the copolymer has a negative segment order parameter (O^(s)) and the copolymer film has a positive birefringence (Δn) that satisfies the relation Δn=Δn^(s)×O^(s)>0.
 93. The method of claim 92 wherein R⁶ is an OASU.
 94. The method of claim 93 wherein R⁶ is a disk.
 95. The method of claim 92 wherein the copolymer comprises at least two different disks.
 96. The method of claim 92 wherein the disk is an aromatic imide or lactam.
 97. The method of claim 96 wherein monomers of the copolymer further comprise styrene.
 98. The method of claim 92 wherein at least one disk is selected from the group consisting of naphthalene, anthracene, pyrene, and phthalimide.
 99. The method of claim 98 wherein monomers of the copolymer further comprise styrene.
 100. The method of claim 92 wherein at least one disk is selected from the group consisting of the following structures, wherein the disk is attached to the polymer backbone via a carbon atom on a benzene ring or a nitrogen atom on an imide or lactam group:


101. The method of claim 100 wherein monomers of the copolymer further comprise styrene.
 102. The method of claim 92 wherein the copolymer is a reaction product of monomers, wherein at least one monomer is selected from the group consisting of:


103. The method of claim 102 wherein the monomers of the copolymer further comprise styrene.
 104. The method of claim 92 wherein at least one ethylenically unsaturated monomer is selected from the group consisting of styrene, vinyl biphenyl, methyl methacrylate, butyl acrylate, acrylic acid, methacrylic acid, acrylonitrile, 2-ethylhexyl acrylate, and 4-t-butylstyrene.
 105. The method of claim 92 wherein the compensation film is capable of forming an out-of-plane anisotropic alignment upon solvent evaporation without being subject to heat treatment, photo irradiation, or stretching and has a positive birefringence greater than 0.002 throughout the wavelength range of 400 nm<λ<800 nm.
 106. The method of claim 92 wherein the compensation film is capable of forming an out-of-plane anisotropic alignment upon solvent evaporation without being subject to heat treatment, photo irradiation, or stretching and has a positive birefringence greater than 0.005 throughout the wavelength range of 400 nm<λ<800 nm.
 107. The method of claim 92 wherein polymer composition is soluble in a solvent selected from the group consisting of toluene, methyl isobutyl ketone, cyclopentanone, and a mixture thereof.
 108. The method of claim 92 wherein polymer composition is soluble in toluene or methyl isobutyl ketone.
 109. The method of claim 92 wherein the compensation film is used in a liquid crystal display device.
 110. The method of claim 109 wherein the liquid crystal display device is an in-plane switching liquid crystal display device.
 111. The method of claim 109 wherein the liquid crystal display device is used as a screen for a television or computer.
 112. A method for controlling positive birefringence in a compensation film for liquid crystal display comprising: (a) determining a value of R/D for a copolymer, wherein the copolymer comprises a moiety:

wherein R¹, R², R³, R⁴, R⁵, and Ware each independently hydrogen atoms, alkyl groups, substituted alkyl groups, or halogens; wherein R⁶ is a hydrogen atom, alkyl, substituted alkyl, halogen, ester, amide, ketone, ether, cyano, phenyl, epoxy, urethane, urea or optically anisotropic subunit (OASU) attached directly to the backbone of the residue of an ethylenically unsaturated monomer; wherein Mesogen is a rod-like optically anisotropic subunit (OASU) attached directly to the polymer backbone via at least one covalent bond, wherein R represents the maximum dimension of the Mesogen in the direction perpendicular to the direction of the vector sum of the at least one covalent bond and D represents the distance along the copolymer backbone between the attaching points of Mesogen and R⁶, and wherein said Mesogen affects the rigidity and long-range linear corkscrew shape of the copolymer backbone such that the average orientation of the Mesogen is perpendicular to the copolymer backbone, and the higher the perpendicularity of the Mesogens, the larger the value of the negative segment birefringence of the copolymer segment; (b) selecting said copolymer as having a controlled negative segment birefringence if said value of R/D>about 2.7; and (c) processing the copolymer having a controlled negative segment birefringence by solution casting onto a substrate, followed by uniaxial stretching or biaxial stretching or a combination thereof, wherein the copolymer has a negative segment order parameter (O^(s)) and the copolymer film has a positive birefringence (Δn) that satisfies the relation Δn=Δn^(s)×O^(s)>0.
 113. The method of claim 112 wherein R⁶ is an OASU.
 114. The method of claim 113 wherein R⁶ is a mesogen.
 115. The method of claim 112 wherein the mesogen has the structure R¹-(A¹-Z¹)_(m)-A²-(Z²-A³)_(n)-R² wherein A¹, A², and A³ are each independently aromatic or cycloaliphatic rings, wherein the rings are all-carbon or heterocyclic, wherein the rings are unsubstituted, mono- or poly-substituted with halogen, cyano or nitro groups or alkyl, alkoxy, or alkanoyl groups having 1 to 8 carbon atoms; wherein Z¹, and Z² are each independently —COO—, —OOC—, —CO—, —CONH—, —NHCO—, —CH═CH—, —C≡C—, —CH═N—, —N═CH—, —N═N—, —O—, —S—, or a single bond; wherein R¹ and R² are each independently halogen, cyano, nitro, or alkyl, alkoxy, or alkanoyl groups having 1 to 25 carbon atoms, or are (Z²-A³) as defined above; wherein m is 0, 1, or 2; and wherein n is 1 or
 2. 116. The method of claim 115 wherein m is 1 or 2, n is 1 or 2, A² is 1,4-phenylene, and the mesogen is attached to the polymer backbone through A².
 117. The method of claim 115 wherein m is 2, n is 2, A² is 1,4-phenylene, and the mesogen is attached to the polymer backbone through A².
 118. The method of claim 112 wherein the copolymer comprises at least two different mesogen groups.
 119. The method of claim 112 wherein at least one mesogen is selected from the group consisting of the following structures, wherein the mesogen is attached to the polymer backbone via a carbon atom on a benzene ring:


120. The method of claim 119 wherein monomers of the copolymer further comprise styrene.
 121. The method of claim 112 wherein at least one mesogen is selected from the group consisting of the following structures, wherein the mesogen is attached to the polymer backbone via a carbon atom on the center 1,4-phenylene:


122. The method of claim 121 wherein monomers of the copolymer further comprise styrene.
 123. The method of claim 112 wherein the copolymer is a reaction product of monomers, wherein at least one monomer is selected from the group consisting of:

wherein the polymer has a positive birefringence greater than about 0.02 throughout the wavelength range of 400 nm<λ<800 nm.
 124. The method of claim 123 wherein the monomers of the copolymer further comprise styrene.
 125. The method of claim 112 wherein the copolymer is a reaction product of monomers, wherein at least one monomer is selected from the group consisting of:


126. The method of claim 125 wherein the monomers of the copolymer further comprise styrene.
 127. The method of claim 112 wherein at least one moiety in the copolymer backbone is selected from the group consisting of the following structures, wherein R¹, R² and R³ are each independently hydrogen, alkyl group, substituted alkyl group or halogen:

wherein the polymer has a positive birefringence greater than about 0.01 throughout the wavelength range of 400 nm<λ<800 nm.
 128. The method of claim 127 wherein monomers of the copolymer further comprise styrene.
 129. The method of claim 112 wherein at least one moiety in the copolymer backbone is selected from the group consisting of the following structures, wherein R¹, R² and R³ are each independently hydrogen, alkyl group, substituted alkyl group or halogen:

wherein the polymer has a positive birefringence greater than about 0.02 throughout the wavelength range of 400 nm<λ<800 nm.
 130. The method of any one of claim 129 wherein monomers of the copolymer further comprises styrene.
 131. The method of claim 112 wherein at least one ethylenically unsaturated monomer is selected from the group consisting of styrene, vinyl biphenyl, methyl methacrylate, butyl acrylate, acrylic acid, methacrylic acid, acrylonitrile, 2-ethylhexyl acrylate, and 4-t-butylstyrene.
 132. The method of claim 112 wherein the compensation film is capable of forming an out-of-plane anisotropic alignment upon solvent evaporation without being subject to heat treatment, photo irradiation, or stretching and has a positive birefringence greater than 0.002 throughout the wavelength range of 400 nm<λ<800 nm.
 133. The method of claim 132 wherein the compensation film is removed from the substrate upon drying to yield a free-standing film.
 134. The method of claim 133, wherein the free-standing film is uniaxially or biaxially stretched.
 135. The method of claim 133, wherein the free-standing film is attached to a substrate by means of lamination.
 136. The method of claim 134, wherein the free-standing film is attached to a substrate by lamination.
 137. The method of claim 112 wherein the compensation film is capable of forming an out-of-plane anisotropic alignment upon solvent evaporation without being subject to heat treatment, photo irradiation, or stretching and has a positive birefringence greater than 0.005 throughout the wavelength range of 400 nm<λ<800 nm.
 138. The method of claim 137 wherein the compensation film is removed from the substrate upon drying to yield a free-standing film.
 139. The method of claim 138, wherein the free-standing film is uniaxially or biaxially stretched.
 140. The method of claim 138, wherein the free-standing film is attached to a substrate by means of lamination.
 141. The method of claim 139, wherein the free-standing film is attached to a substrate by lamination.
 142. The method of claim 112 wherein the compensation film is capable of forming an out-of-plane anisotropic alignment upon solvent evaporation without being subject to heat treatment, photo irradiation, or stretching and has a positive birefringence greater than 0.01 throughout the wavelength range of 400 nm<λ<800 nm.
 143. The method of claim 142 wherein the compensation film is removed from the substrate upon drying to yield a free-standing film.
 144. The method of claim 143, wherein the free-standing film is uniaxially or biaxially stretched.
 145. The method of claim 143, wherein the free-standing film is attached to a substrate by means of lamination.
 146. The method of claim 144, wherein the free-standing film is attached to a substrate by lamination.
 147. The method of claim 112 wherein the compensation film is capable of forming an out-of-plane anisotropic alignment upon solvent evaporation without being subject to heat treatment, photo irradiation, or stretching and has a positive birefringence greater than 0.02 throughout the wavelength range of 400 nm<λ<800 nm.
 148. The method of claim 147 wherein the compensation film is removed from the substrate upon drying to yield a free-standing film.
 149. The method of claim 148, wherein the free-standing film is uniaxially or biaxially stretched.
 150. The method of claim 148, wherein the free-standing film is attached to a substrate by means of lamination.
 151. The method of claim 149, wherein the free-standing film is attached to a substrate by lamination.
 152. The method of claim 112 wherein the compensation film is capable of forming an out-of-plane anisotropic alignment upon solvent evaporation without being subject to heat treatment, photo irradiation, or stretching and has a positive birefringence greater than 0.03 throughout the wavelength range of 400 nm<λ<800 nm.
 153. The method of claim 152 wherein the compensation film is removed from the substrate upon drying to yield a free-standing film.
 154. The method of claim 153, wherein the free-standing film is uniaxially or biaxially stretched.
 155. The method of claim 153, wherein the free-standing film is attached to a substrate by means of lamination.
 156. The method of claim 154, wherein the free-standing film is attached to a substrate by lamination.
 157. The method of claim 112 wherein polymer composition is soluble in a solvent selected from the group consisting of toluene, methyl isobutyl ketone, cyclopentanone, and a mixture thereof.
 158. The method of claim 112 wherein polymer composition is soluble in toluene or methyl isobutyl ketone.
 159. The method of claim 112 wherein the compensation film is used in a liquid crystal display device.
 160. The method of claim 159 wherein the liquid crystal display device is an in-plane switching liquid crystal display device.
 161. The method of claim 159 wherein the liquid crystal display device is used as a screen for a television or computer.
 162. A method for controlling positive birefringence in a compensation film for liquid crystal display comprising: (a) determining a value of R/D for a polymer, wherein the polymer comprises a polymer segment having a polymer backbone and a light-stable optically anisotropic sub-unit (OASU) comprising an aromatic ring and at least one birefringence enhancing substituent (BES) attached to the aromatic ring, wherein the Ar-BES is attached directly to the polymer backbone via at least one covalent bond, wherein R represents the maximum dimension of the Ar-BES in the direction perpendicular to the direction of the vector sum of the at least one covalent bond and D represents the distance along the polymer backbone between the attaching points of two neighboring Ar-BESs, and wherein Ar-BES affects the rigidity and long-range linear corkscrew shape of the polymer backbone such that the average orientation of the Ar-BES is perpendicular to the polymer backbone, and the higher the perpendicularity of the Ar-BESs, the larger the value of the negative segment birefringence of the polymer; (b) selecting said polymer as having a controlled negative segment birefringence if said value of R/D>about 2.6; and (c) processing the polymer having a controlled negative segment birefringence (Δn^(s)) by solution casting onto a substrate, followed by uniaxial stretching or biaxial stretching or a combination thereof, wherein the polymer has a negative segment order parameter (O^(s)) and the polymer film has a positive birefringence (Δn) that satisfies the relation Δn=Δn^(s)×O^(s)>0.
 163. The method of claim 162 wherein the polymer comprises a moiety of

wherein R₁, R₂, and R₃ are each independently hydrogen atoms, alkyl groups, substituted alkyl groups, or halogens; Ar is an aromatic ring; and BES represents at least one birefringence enhancing substituent.
 164. The method of claim 162 wherein the polymer is a homopolymer.
 165. The method of claim 164 wherein the compensation film is capable of forming an out-of-plane anisotropic alignment upon solvent evaporation without being subject to heat treatment, photo irradiation, or stretching and has a positive birefringence greater than 0.002 throughout the wavelength range of 400 nm<λ<800 nm.
 166. The method of claim 165 wherein the compensation film is removed from the substrate upon drying to yield a free-standing film.
 167. The method of claim 166, wherein the free-standing film is uniaxially or biaxially stretched.
 168. The method of claim 166, wherein the free-standing film is attached to a substrate by means of lamination.
 169. The method of claim 167, wherein the free-standing film is attached to a substrate by lamination.
 170. The method of claim 164 wherein the compensation film is capable of forming an out-of-plane anisotropic alignment upon solvent evaporation without being subject to heat treatment, photo irradiation, or stretching and has a positive birefringence greater than 0.005 throughout the wavelength range of 400 nm<λ<800 nm.
 171. The method of claim 170 wherein the compensation film is removed from the substrate upon drying to yield a free-standing film.
 172. The method of claim 171, wherein the free-standing film is uniaxially or biaxially stretched.
 173. The method of claim 171, wherein the free-standing film is attached to a substrate by means of lamination.
 174. The method of claim 172, wherein the free-standing film is attached to a substrate by lamination.
 175. The method of claim 164 wherein the compensation film is capable of forming an out-of-plane anisotropic alignment upon solvent evaporation without being subject to heat treatment, photo irradiation, or stretching and has a positive birefringence greater than 0.01 throughout the wavelength range of 400 nm<λ<800 nm.
 176. The method of claim 175 wherein the compensation film is removed from the substrate upon drying to yield a free-standing film.
 177. The method of claim 176, wherein the free-standing film is uniaxially or biaxially stretched.
 178. The method of claim 176, wherein the free-standing film is attached to a substrate by means of lamination.
 179. The method of claim 177, wherein the free-standing film is attached to a substrate by lamination.
 180. The method of claim 164 wherein polymer composition is soluble in a solvent selected from the group consisting of toluene, methyl isobutyl ketone, cyclopentanone, and a mixture thereof.
 181. The method of claim 180 wherein BES is nitro- or bromo-.
 182. The method of claim 164 wherein polymer composition is soluble in a solvent selected from the group consisting of toluene or methyl isobutyl ketone or a mixture thereof.
 183. The method of claim 182 wherein BES is nitro- or bromo-.
 184. The method of claim 164 wherein the compensation film is used in a liquid crystal display device.
 185. The method of claim 184 wherein the liquid crystal display device is an in-plane switching liquid crystal display device.
 186. The method of claim 184 wherein the liquid crystal display device is used as a screen for a television or computer.
 187. The method of claim 164 further comprising controlling the degree of substitution of the BES by adjusting starting amounts of BES in a reaction mixture.
 188. The method of claim 164 wherein the degree of substitution of the BES is greater than 0.5.
 189. The method of claim 164 wherein the BES is selected from the group consisting of nitro-, bromo-, iodo-, cyano- and phenyl-.
 190. The method of claim 189 wherein the BES is nitro- or bromo-.
 191. The method of claim 164 wherein the aromatic ring is selected from the group consisting of benzene, biphenyl, naphthalene, anthracene, phenanthrene, naphthacene, pentacene, and triphenyl.
 192. The method of claim 191 wherein the aromatic ring is benzene.
 193. The method of claim 164 wherein BES is nitro- or bromo- and the aromatic ring is selected from the group consisting of benzene, biphenyl, naphthalene, anthracene, phenanthrene, naphthacene, pentacene, and triphenyl.
 194. The method of claim 164 wherein the polymer is poly(nitrostyrene).
 195. The method of claim 194 wherein the poly(nitrostyrene) has a positive birefringence greater than 0.007 throughout the wavelength range of 400 nm<λ<800 nm.
 196. The method of claim 195 wherein the poly(nitrostyrene) has a degree of substitution greater than 0.7 for the nitro group.
 197. The method of claim 194 wherein the poly(nitrostyrene) has a degree of substitution greater than 0.5 for the nitro group.
 198. The method of claim 194 wherein the poly(nitrostyrene) has a degree of substitution greater than 0.7 for the nitro group.
 199. The method of claim 194 wherein BES comprises para-nitro groups.
 200. The method of claim 194 wherein BES consists of para-nitro groups.
 201. The method of claim 200 wherein the poly(nitrostyrene) has a degree of substitution greater than 0.7 for the nitro group.
 202. The method of claim 200 wherein the poly(nitrostyrene) has a degree of substitution greater than 0.5 for the nitro group.
 203. The method of claim 164 wherein the polymer is poly(bromostyrene).
 204. The method of claim 203 wherein the poly(bromostyrene) has a positive birefringence greater than 0.005 throughout the wavelength range of 400 nm<λ<800 nm.
 205. The method of claim 204 wherein the poly(bromostyrene) has a degree of substitution greater than 0.7 for the bromo group.
 206. The method of claim 203 wherein the poly(bromostyrene) has a degree of substitution greater than 0.5 for the bromo group.
 207. The method of claim 203 wherein the poly(bromostyrene) has a degree of substitution greater than 0.7 for the bromo group.
 208. The method of claim 203 wherein BES comprises para-bromo groups.
 209. The method of claim 203 wherein BES consists of para-bromo groups.
 210. The method of claim 209 wherein the poly(bromostyrene) has a degree of substitution greater than 0.5 for the bromo group.
 211. The method of claim 209 herein the poly(bromostyrene) has a degree greater than 0.7 for the bromo group.
 212. A method for controlling positive birefringence in a compensation film for liquid crystal display comprising: (a) determining a value of R/D for a copolymer, wherein the copolymer comprises a moiety:

wherein R¹, R², R³, R⁴, R⁵, and R⁷ are each independently hydrogen atoms, alkyl groups, substituted alkyl groups, or halogens; wherein R⁶ is a hydrogen atom, alkyl, substituted alkyl, halogen, ester, amide, ketone, ether, cyano, phenyl, epoxy, urethane, urea or optically anisotropic subunit (OASU) attached directly to the backbone of the residue of an ethylenically unsaturated monomer; wherein Ar-BES is an aromatic ring (Ar) substituted with at least one birefringence enhancing substituent (BES), wherein Ar-BES is attached directly to the copolymer backbone via at least one covalent bond, wherein R represents the maximum dimension of the Ar-BES in the direction perpendicular to the direction of the vector sum of the at least one covalent bond and D represents the distance along the copolymer backbone between the attaching points of Ar-BES and R⁶, and wherein Ar-BES affects the rigidity and long-range linear corkscrew shape of the copolymer backbone such that the average orientation of the Ar-BES is perpendicular to the copolymer backbone, and the higher the perpendicularity of the Ar-BESs, the larger the value of the negative segment birefringence of the copolymer; (b) selecting said copolymer as having a controlled negative segment birefringence if said value of R/D being at least about 2.6; and (c) processing the copolymer having a controlled negative segment birefringence (Δn^(s)) by solution casting onto a substrate, followed by uniaxial stretching or biaxial stretching or a combination thereof, wherein the copolymer has a negative segment order parameter (O^(s)) and the copolymer film has a positive birefringence (Δn) that satisfies the relation Δn=Δn^(s)×O^(s)>0.
 213. The method of claim 212 wherein R⁶ is an OASU.
 214. The method of claim 213 wherein R⁶ is an Ar-BES.
 215. The method of claim 212 wherein the copolymer comprises at least two different Ar-BES groups.
 216. The method of claim 212 wherein at least one ethylenically unsaturated monomer is selected from the group consisting of styrene, vinyl biphenyl, methyl methacrylate, butyl acrylate, acrylic acid, methacrylic acid, acrylonitrile, 2-ethylhexyl acrylate, and 4-t-butylstyrene.
 217. The method of claim 212 wherein at least one monomer of the copolymer further comprises styrene.
 218. The method of claim 212 wherein the compensation film is capable of forming an out-of-plane anisotropic alignment upon solvent evaporation without being subject to heat treatment, photo irradiation, or stretching and has a positive birefringence greater than 0.002 throughout the wavelength range of 400 nm<λ<800 nm.
 219. The method of claim 218 wherein the compensation film is removed from the substrate upon drying to yield a free-standing film.
 220. The method of claim 219, wherein the free-standing film is uniaxially or biaxially stretched.
 221. The method of claim 219, wherein the free-standing film is attached to a substrate by means of lamination.
 222. The method of claim 220, wherein the free-standing film is attached to a substrate by lamination.
 223. The method of claim 212 wherein the compensation film is capable of forming an out-of-plane anisotropic alignment upon solvent evaporation without being subject to heat treatment, photo irradiation, or stretching and has a positive birefringence greater than 0.005 throughout the wavelength range of 400 nm<λ<800 nm.
 224. The method of claim 223 wherein the compensation film is removed from the substrate upon drying to yield a free-standing film.
 225. The method of claim 224, wherein the free-standing film is uniaxially or biaxially stretched.
 226. The method of claim 224, wherein the free-standing film is attached to a substrate by means of lamination.
 227. The method of claim 225, wherein the free-standing film is attached to a substrate by lamination.
 228. The method of claim 212 wherein the compensation film is capable of forming an out-of-plane anisotropic alignment upon solvent evaporation without being subject to heat treatment, photo irradiation, or stretching and has a positive birefringence greater than 0.01 throughout the wavelength range of 400 nm<λ<800 nm.
 229. The method of claim 228 wherein the compensation film is removed from the substrate upon drying to yield a free-standing film.
 230. The method of claim 229, wherein the free-standing film is uniaxially or biaxially stretched.
 231. The method of claim 229, wherein the free-standing film is attached to a substrate by means of lamination.
 232. The method of claim 230, wherein the free-standing film is attached to a substrate by lamination.
 233. The method of claim 212 wherein polymer composition is soluble in a solvent selected from the group consisting of toluene, methyl isobutyl ketone, cyclopentanone, and a mixture thereof.
 234. The method of claim 233 wherein BES is nitro- or bromo-.
 235. The method of claim 212 wherein polymer composition is soluble in a solvent selected from the group consisting of toluene or methyl isobutyl ketone or a mixture thereof.
 236. The method of claim 235 wherein BES is nitro- or bromo-.
 237. The method of claim 212 wherein the compensation film is used in a liquid crystal display device.
 238. The method of claim 237 wherein the liquid crystal display device is an in-plane switching liquid crystal display device.
 239. The method of claim 237 wherein the liquid crystal display device is used as a screen for a television or computer.
 240. The method of claim 212 further comprising controlling the degree of substitution of the BES.
 241. The method of claim 212 wherein the degree of substitution of the BES is greater than 0.5.
 242. The method of claim 212 wherein the BES is selected from the group consisting of nitro-, bromo-, iodo-, cyano- and phenyl-.
 243. The method of claim 242 wherein the BES is nitro- or bromo-.
 244. The method of claim 212 wherein the aromatic ring is selected from the group consisting of benzene, biphenyl, naphthalene, anthracene, phenanthrene, naphthacene, pentacene, and triphenyl.
 245. The method of claim 244 wherein the aromatic ring is benzene.
 246. The method of claim 212 wherein BES is nitro- or bromo- and the aromatic ring is selected from the group consisting of benzene, biphenyl, naphthalene, anthracene, phenanthrene, naphthacene, pentacene, and triphenyl.
 247. The method of claim 212 wherein the polymer is poly(nitrostyrene-co-styrene).
 248. The method claim 247 wherein the poly(nitrostyrene-co-styrene) has a degree of substitution greater than 0.5 for the nitro group.
 249. The method of claim 247 wherein the poly(nitrostyrene-co-styrene) has a degree of substitution greater than 0.7 for the nitro group.
 250. The method of claim 247 wherein BES comprises para-nitro groups.
 251. The method of claim 247 wherein BES consists of para-nitro groups.
 252. The method of claim 251 wherein the poly(nitrostyrene-co-styrene) has a degree of substitution greater than 0.5 for the nitro group.
 253. The method of claim 251 wherein the poly(nitrostyrene-co-styrene) has a degree of substitution greater than 0.7 for the nitro group.
 254. The method of claim 212 wherein the polymer is poly(bromostyrene-co-styrene).
 255. The method of claim 254 wherein the poly(bromostyrene-co-styrene) has a degree of substitution greater than 0.5 for the bromo group.
 256. The method of claim 254 wherein the poly(bromostyrene-co-styrene) has a degree of substitution greater than 0.7 for the bromo group.
 257. The method of claim 254 wherein BES comprises para-bromo groups.
 258. The method of claim 254 wherein BES consists of para-bromo groups.
 259. The method of claim 258 wherein the poly(bromostyrene-co-styrene) has a degree of substitution greater than 0.5 for the bromo group.
 260. The method of claim 258 wherein the poly(bromostyrene-co-styrene) has a degree of substitution greater than 0.7 for the bromo group.
 261. The method of any of claims 1, 92, 112, 162 and 212, wherein R/D is at least
 3. 262. The method of claim 261, wherein R/D is at least
 4. 263. The method of claim 261, wherein R/D is at least
 6. 